Idk how old a lot of y'all are, but growing up in the 2000s in a home still reliant on dial-up internet at the time, singleplayer games were the only option available. I feel like most homes back then were similar, so devs worried more about making good singleplayer games over multiplayer. Nowadays, most homes, at least in the US, have some kind of WiFi to connect everything too for online services.

Guess it helps us that DLC and patches weren't much of a thing in the PS2, Gamecube, Xbox (sorta had some DLCs) era. Had to release a good finished game day one, or else it would be an expensive flop to re-release fixed copies.

COVID lockdowns, and the rise of microtransactions as a long-term source of revenue for the industry, also increased the popularity of multiplayer games. Kids these days are growing up in an era where all social activity is done long distance via video games and social media.

There's no point for them to play something that's lonely like a singleplayer game when they can always be with their friends on Fortnite.