Where does this compulsion come from that people feel like they have to go out, buy every book...
For me, two reasons:
- In both 3e and 4e (and, I expect, 5e as well), there were clear parts of the game that were ideal for expansion. Although the core rules of both were complete, it was clear that 3e could benefit from many more feats, while 4e could use more powers (and, in both cases, other areas as well, but those were the two that leapt out at me).
- If I'm actively running the game, I actually do like supporting the company who produce it. If nothing else, I want to encourage them to make more.
Actually, I never did. For the overwhelming majority of the time I ran 3e, it was either "core rules only" or "this handful of books" only. Still, if I'm spending all that money on books, I do actually want to use them!
and then complain about it?
I do
try not to complain about the
existence of those supplements. However, I do think it's fair to complain about the
quality of those supplements, which was often not what it should be.
I did also find, partly due to that poor quality, partly due to deliberate power-creep, partly due to an expansion in complexity over time, and partly due to emergent issues, adding lots of supplements to a game actually made the game
worse - and that was true even if you maintained rigid control over what you allowed! The sheer weight of those supplementary rules, on top of an already very heavy system made 3e, in particular, less than enjoyable to run.
People talk about supplement books drive people away because of their bloat.
Actually, I think that's a known phenomenon - if a new player goes into his FLGS and sees row upon row of books, he's less likely to invest in the core rulebook for the game as a consequence. This is true even if all those supplements are clearly marked as such, but when they're given unhelpful names like "Player's Handbook 2", or the back-cover text describes it as a "must-have accessory", that scares the new guy away.
The biggest thing for Wizards is just to make sure they come out with quality content.
Absolutely.