They would be able to attack from a much greater distance and it would definitely decrease the risk of having their limbs chopped off by other lightsaber users.
Telekinesis requires concentration/mental resources. Watch the battle between Obi-Wan and Vader in Ep. 3 and Rey and Kylo's struggle over Anakin's lightsaber in Ep. 8. In Ep. 5, Luke almost needs to meditate before he is able to lift his lightsaber using telekinesis when he is trapped in the cave on Hoth. Even Yoda, a well-experienced master, needs a second or two to levitate repulsor platforms in the Senate in Ep. 3.
Although Yoda says, "Do or do not, there is no try", it is clear that "doing" is not free in terms of mental or physical stress. Luke's tricky Force trick (albeit not telekinesis per se) near the end of Ep. 8, and his subsequent exhaustion, demonstrate that using the Force is an involved process that is more than simply willing something done.
If Force users were to use telekinesis for all battles, it would drain them more than standard lightsaber combat would, making them comparatively worse fighters and perhaps making it more difficult for them to watch for other troubles.
If telekinesis were "free", why didn't Luke in Ep. 8 simply use telekinesis to throw Kylo, Snoke, or perhaps the entire First Order into a Rancor pit (or a black hole, or into a star, or wherever) and be done?
I'm sure this is a possibility for Force-users, but as a previous answer has said, that would require quite an intense amount of concentration for the person choosing to wield their saber in this way. Also, on top of that, you'd have to be extremely careful to keep the lightsaber actually ignited the entire time it's flying through the air. There are pressure plates on those things that ensure that it turns off when someone isn't holding it anymore, and in order to keep it on without holding it, you would have to maintain an extremely precise amount of pressure through the Force, while also directing where the saber is flying. That could get pretty tricky when you've already got at least one other person hoping to lop off a limb or two, or, more probably, your head.
However, a situation similar to what you described has happened several times in canon before, as was mentioned with Vader throwing his saber at Luke, as is actually a fairly frequently used trick by the Imperial Inquisitors in Star Wars: Rebels' first couple of seasons. The Inquisitors' lightsabers are double-sided AND spin, and seem to have been specially constructed with the idea of using them as a "saber frisbee/boomerang" of sorts in mind. The Grand Inquisitor especially often resorts to throwing his saber (while in spinning mode) at his enemies, which can get a little scary. But the Jedi don't seem to have a ton of trouble avoiding this, and usually just bat it away with their own sabers. So this is another reason why it doesn't seem to be relied upon in actual battle circumstances.
Another reason against this maneuver would be that, once you'd thrown your lightsaber at your opponent, if they somehow managed to avoid it and get on close quarters with you, you're stuck with no way to defend yourself. It's very likely that they would be able to dodge/deflect a flying saber, especially if they Force-pushed it away. And it would take time, energy, and focus to be able to locate your saber wherever it has gotten off to and call it back to you - you probably wouldn't be able to achieve all that, plus get into a good position to defend yourself, if you have an enemy already on top of you. Also, if you've thrown your saber at them, they could easily try to Force-grab it away, and then it just turns into one of those tug-of-war moments we're seeing Kylo and Rey get into all the time.
Now you could definitely employ this technique if you were wielding dual sabers, or maybe even if you're using it as a last-ditch effort to take down a fleeing opponent, or someone with no defenses. But that's not combat, and I got the impression from your original question that you were primarily asking about times when multiple Force-users/lightsaber-wielders were fighting each other.
But if you aren't specifically talking about lightsaber combat situations and are referring to times when, as I just said, you're attempting to take down a fleeing or unarmed opponent, then yes, it could be a great technique to resort to, and I feel like we've seen it before. But it is a very Sith sort of thing to do. (Actually, I think Maul did something like that in the Star Wars Rebels season 2 finale, "Twilight of the Apprentice" when he killed the Seventh Sister, but I'm not completely positive, so don't anyone quote me on that...)