The sad thing is enterable buildings is pretty achievable. They could have an algorithm to generate a generic interior based on a seed at the time that the player enters the building. It's not exciting, and it would add complexity but it at least would make enterable buildings without massively impacting install size nor save size.

You can play with procedural generated cities and structures here for example to see the potential. Just imagine that instead of a top down map it generates an interior with generic furniture, clutter and possibly loot. Add in different sets of assets it fills in with based on building type and location and verify it generates the same interior every time it gets the same seed and you have a very efficient system for at least being able to enter every random boring building

Alternatively there is a gameplay argument to be made for only being able to enter buildings that actually do something gameplay-wise so you know what buildings you need to enter