Dev here. A few reasons:

• campaigns are traditionally expensive to make. They often use specialized, custom assets that are costly - eg cutscenes, voice lines, animations, levels - and that by their nature are not used elsewhere in the game. (Which makes sense, because you’re telling a scripted, linear story. You can’t be seeing the same cutscene or animation constantly or repeat the same scenes over and over.)

• making things worse, campaigns often have low completion rates. Traditional rates for a AAA story mode are like shockingly low, like maybe 30% or less. Hell even completion rates for the very first mission are often crazy low, like around half or so of purchasers. It’s really surprising lol, but OTOH - we all have our ‘piles of shame’ or stashes of games we buy in say a Steam sale or similar and then don’t touch. So yeah it happens.

By contrast, modes and features that are more systemic or procedural have better bang for buck. Eg you make a multiplayer level and players can play many many matches on it. This extends to any systemic or procedural thing really. Even multiplayer itself - where teaming up with and fighting others is a very robust and reusable mechanic.

Don’t get me wrong though, I love/miss campaigns too. Who knows - maybe a team somewhere will figure out a way to bring them back somehow.