Your generalization doesn’t hold. Take Cities: Skylines (a city building game; compare it to Sim City) as an example. The base game cost €30 at launch [1]. It’s a (kinda [2]) functional base game, however, it’s somewhat flat.

If you’re interested in more challenge building industries (instead of just zoning industrial zones and that’s it), you buy the industries DLC (currently €15) where you need to juggle supply chains. If you’re bored by just plopping down some parks to make people happy, you buy the Parklife DLC (also €15), which allows you to be more creative in providing recreation opportunities. If you’re an old Transport Tycoon player and want to create the perfect public transport network, buy the Mass Transit DLC (€13).

The base game is fully functional without all these DLCs, and each one focuses on an aspect of the game into which some players might want to dig deeper, but not others.

[1] According to https://steampricehistory.com/app/255710 [2] I say “kinda” because it does have flaws. However, these aren’t fixed in DLCs, so my point still stands.