LEGO kits used to be composed of fully interchangeable parts. While that is still true of the core build, there are now unique parts in every kit. This is driven by sales.
Nearly all of the parts are designed around a small set of common interfaces. The main change is that have moderately increased the number of interfaces vs. a few decades ago and there are more small decorative parts (and thus higher overall part counts) in most large current sets.
Designing models that are comparable to the first-party models is a more difficult challenge for kids than it used to be, but there is also a much larger pool of extremely skilled builders than there used to be (both adults and children). But there’s nothing stopping anyone from using the current pieces in older-style projects, and the older pieces couldn’t be used in quite as large a variety of models as the current ones.
