Actually, there is a laser-class weapon in books 7 and 8, used by Laconian Magnetar-class battlecruisers.
In book 7, it's firing on the central sphere in the gate space causes an outburst of gamma radiation from all gates, capable of frying entire ships at significant distances. Similarly, in book 8, a gamma radiation outburst of a neutron star turning into black hole, directed at the gate (and consecutively, at the sphere), causes an identical effect, only way stronger (it doesn't just fry ships anymore, it vaporizes them instantly this time, just like the main gun discharge on a Magnetar-class battlecruiser does). So, even though it hasn't been stated explicitly, the conclusion is that those ships' primary weapon is basically a very big gamma ray laser.
It has to be taken into consideration, though, that this armament is extremely power-consuming. In book 8, it is stated that the Resistance, up until then, didn't figure out what power source could be powerful enough to feed this weapon. Until, in a raid on a Laconian freighter, Bobbie's team discovers four containment cells, that is. It turns out they contain a kilogram of antimatter either (as far as I remember) in total, or each - and that discovery puts the final piece of the aforementioned puzzle in place. Judging by how The Heart of Tempest made only a few shots and needed resupply, we can assume that an entire containment cell's content is annihilated to power a single discharge.
EDIT: Now I've come, however, to conclusion that while the statement above does somehow answer the question in letter, it does not answer it in spirit. Which I assume was 'why lasers are not widely used by UN/MCR/OPA ships'
In that regard, I recognize the technical arguments presented by other users, though they might not be completely valid. Now, what do I mean by that? In terms of laser as a main artillery weapon, the arguments on power output of a laser (more precisely, it's input/output efficiency) and the mobility of targets seem largely valid. However, as PDC replacements, they do not.
The US navy is implementing such defences on their Ford class supercarriers right now. Those are powered by nuclear reactors, ones with similar output or marginally higher than those powering Zumwalt class destroyers, which are either already equipped or are planned to be equipped with railguns. Ones that accelerate projectiles to hypersonic velocities. Now, from what I read, railguns like the ones on Donnager or Truman class accelerate their projectiles to a measurable fraction of light speed. That hints at their reactors providing more power by at least an order of magnitude (1000x).
Now, what has to be taken into account - in space, power loss of a laser beam due to photon deflection is lower over any measured distance by a lot due to vastly lower particle density (actually, negligible, at least when we are talking about distances measured in light seconds which are many times greater than distances at which battles in The Expanse actually take place). And still, despite higher energy loss and lower available power input, those PD lasers are being implemented here, on Earth, nowadays. Now, would they be implemented if they were inferior to projectile CIWS which are basically a predecessor to The Expanse's PDC? Doubtful.
Let's say that aforementioned is a 'currently used weapon' argument.
It is stated in the books that even communication lasers can do a lot of damage, especially to peripheral devices on a ship. Which means ones that due to their function cannot be protected by a layer of armor while at use. Such as sensors, communications equipment... or perhaps even active PDCs.
One should remember that while these lasers can be weaponized at the cost of shorter lifespan or even melting risk due to power input far beyond the intended one for them... they are not purpose-built weapons. Those would most definitely have been built with higher input and better cooling in mind. And probably not powered from reactor directly, but using capacitors to allow discharging a second or ten seconds of power feed within a milisecond or even microsecond (like Honorverse lasers do).
Now, why I think lasers could be employed in The Expanse universe's reality as weapons? Precisely because of what they can do to critical, unarmored devices. Does a missile have to be destroyed in order to count it off? Or is it enough to fry its' fragile sensors (that just can not be armored or they would not work) and guidance and just let it miss. What good does having a railgun do to a ship that is blind and doesn't know where to aim its' weapon? What good will instantly overheated or maybe even melted PDCs do? Hull intact, ship crippled, devoid of combat capability, out of commission. The beam might even be slightly dispersed on purpose to be able to catch multiple missiles at once or cover an entire ship if the output was powerful enough to begin with.
With a lot more powerful reactors than we have nowadays, the fact PD lasers are already being implemented and the fact we already have a petawatt laser on Earth, purpose-built laser weapons seem absolutely feasible in The Expanse's reality.
<<OPINION HAZARD, IF YOU ARE SENSITIVE TO THAT, I ADVISE TO CEASE READING AT THIS POINT>>
Well... if that is the case and lasers were there and they were used like I mentioned above, Roci probably wouldn't survive book 2, namely that assault on Protogen station. In pitched battles, ships would have to launch dozens of missiles to have a chance of a single hit, battles would be won and lost within seconds with no place for skillful maneuvering, just a game of sensors, ECM and ECCM - who shot first, who got more accurate lock on target, who had better decoys and/or stealth. Fleet that got a jump on their enemies would, in most cases, win without a single loss. Without defenses that do not seem scientifically feasible on The Expanse's 'level' of technological advancement like energy shields or adaptive-reflective armor, lasers would break the story.
So, in my opinion, their story-breaking nature resulting from author's goal to make a story without an abundance of 'space magic', like shields is the primary, underlying reason they're not there.