You can still buy “generic” lego sets if you want. Look for “Lego Classic” sets.

They're a bit too simplistic though.

Some of the classic 80s themes, like Space and Castle, primarily used regular bricks of reasonable sizes in a very limited palette of colours, with a few special parts unique to the theme. They were much more suited to taking apart and building your own creations.

These days, there's just too many specialised and small parts, and too many colours. Even if you buy a big grey Star Wars set, you'll find that the internal structure is often brightly coloured to make the instructions clearer - but this isn't ideal if you want to take it apart and build something else.