tl;dr They kill demons because it's faster, easier, safer long-term, and their experience has shown that they don't really gain much by doing exorcisms (as the demon hosts usually die anyway.)
Killing a demon with the knife is much faster, quiter, and easier than performing a ritual. The ritual requires them to keep the demon in one place long enough to finish chanting, which usually means tricking them into a devil's trap or overpowering them physically. In the early episodes, as mentioned in the comments, this was the only option, so they had to take it. Once they had better options available, it became a strategic decision, one that usually leads to stabbiness.
Note that, when possible, they have opted for exorcism instead of death; for example, they lured an entire town full of people into a police station and performed a mass exorcism during Season Four. Also, when Sam had his magic powers, he did explicitly point out that he preferred to use them because his victims sometimes lived, while the knife always killed them.
In the early seasons the demons were mostly a nuisance, and in very low numbers, so it was fine to just send them back to hell individually. Once they started a war against Lilith, then later Lucifer, and at present, Crowley, demons became the foot soldiers in that war. There were a lot more of them around, and the brothers spend a lot of time just trying to avoid them. In that case, sending a demon back to hell is dangerous and counter-productive; it will just inform the boss-of-the-season where the Winchesters are.
(Dean makes this exact point in the second episode of season 8 - Sam actually does a reverse exorcism to keep a demon in its human host so Dean can kill it, because they don't want it running to Crowely with their location.)
Lack of Payoff
One of the things the boys seem to resign themselves to in later seasons is that the human hosting the demon is already dead and gone. In large part this is due to their own attitudes becoming darker, as well as a general fatigue with the whole demon battle scenario. Having killed probably hundreds of demons, a single one no longer registers on their radar. This is a pretty dramatic turn from Season 2 where Bobby actually stops Dean from hitting a possessed Meg because she's human.
But there's also a sense that trying to save the host isn't worth the trouble. Even Sam admits that his power, which doesn't physically harm the host at all, only "might" leave them alive. As we saw with Meg, just because the demon is exorcised successfully doesn't mean the host survives; the demon could destroy the host on purpose, or they could have suffered a fatal injury already, or they could just have aged beyond their natural life span while possessed. The comments that they (Dean particularly) make about possessed people in later seasons seems to reflect this type of experience -- that the people they managed to "save" are dead anyway, so they are adding a lot of risk without really saving anyone.