Games are more expensive to make and prices have not kept up with inflation, but units sold are at least two orders of magnitude than they were twenty years ago, and individual free-to-play titles have playercounts that dwarf the total userbase of all games at the turn of the millennium.
Multiplayer does not require constant funding, unless you want to lock the game down so you can sell microtransactions. I can fire up the original Quake or UT99, pick a server, and play online; it doesn't cost id a cent. On the other hand, I recently tried playing Splinter Cell: Blacklist again, a game released in 2013, and their centralized online services don't work.
Developers have made an effort to keep players from hosting their own servers, which has introducing many new problems. For example, the lack of community moderation / 'toxicity', homogenization of gameplay options within a title to support a matchmaking pool, and stagnation due to lack of new maps, modes, and mods that used to be created by the community.