A lot of games switched to one-book formats in the 90's, thinking about it, although as they got more successful, many more splatbooks followed (FASA's Shadowrun and Earthdawn, Vampire the Masquerade). I never really gave much thought to D&D having three books because, well, it had been that way for a long time. It wasn't until, again, Pathfinder 1e that I was like "now wait, I can buy this one big book for 50 bucks, where WotC is charging me 30 bucks a pop?" that I actually started to feel like I was being fleeced, lol. What was I paying for? The cover?
It got even crazier when Paizo was like "you can access almost everything in our game online for free, and hey, want to buy our books for cheap in pdf form?". Which wasn't perfect, like when they edited my digital copy of the Advanced Class Guide without me realizing it, lol, because a cool feat printed therein was too spicy for their precious Pathfinder Society right before my Oracle could take it...but it was a breath of fresh air.
Of course, like any true gamer, any money I saved on books was immediately spent on other books, so...