There's nothing stopping you from still building random stuff from Legos. In fact there are Lego Classic boxes that are intended just for that.

I think it comes down to personality and interest. One of my kids just buys the occasional Lego and builds it exactly per spec. The other will do that too, but also will dig into our buckets of extra legos from abandonded sets of the past to build cool stuff (a mech, a Magic card stand, a Minecraft sign for his room) or mod his sets by incorporating other Legos to add weapons or decorations, make the limbs longer, etc.

I think the analogy holds quite well for D&D: there are people who just want to run a pre-written adventure, and other people who are crafting their own worlds and scenarios, but will happily mine published materials for ideas.

There is nothing stopping anyone. I am just saying that there is a clear change in behavior of playing with Lego from two decades ago to now. A look at the assortment of a Lego store should make that clear.

Buckets of bricks are much more usable now than they used to be - because of the wealth of different shapes now available. You can still build crude blocky houses, but you don't have to stop there. There are still some incoming overly specialized parts but not all that many these days - and perhaps we have gotten more creative in how to use them.

The current manuals assemble models with complex assembly methods (with the human kind of just a tool in the process). One things that's lacking from the current manuals is an isolation or demonstration of which parts are these clever sub-assemblies and how they can be used in more general situations. That is a problem. You end up with clever constructions methods that are explained on youtube videos and 3rd party books - rather than in the manuals. Missed opportunity.

Another thing that's likely is that with "shelf models" like the botanicals or some Star Wars kits, the kits are bought and gifted to people who have no inclination to be endlessly disassembling and rebuidling different things with them. That's okay - eventually these bricks will end up in our buckets.

who needs creativity when you can have built small-scale versions of your favorite intellectual properties?

The first LEGO set I had as a kid was https://brickset.com/sets/325-3/Shell-Service-Station (issued 1966) with a special-purpose baseplate, doors, sign, gas pumps, and entirely non-LEGO-compatible tank truck.

I have always done both. Both as a child and now with my children. We buy the sets and build them when they are new. They stay intact for a while, but eventually they get extended, rebuilt and eventually join entropy in a big box. Then from the entropy box we build completely new stuff: space ships, air planes, castles and cars. I love stuff with moving parts. Secret doors, robotic arms, escape pods…

I always assumed this is what everyone does.

This is what my brother and I did too. (My sister was young enough to mostly experience the "entropy box.") If I had to guess, I'd imagine a plurality of lego players do it that way, if not a majority, with smaller groups of people exclusively building from instructions or rarely building from instructions.

I got into a routine of buying a (small) set every weekend with my child, when he was about five years old.

The moment he first built a model from scratch, following the instructions and without needing me to help, was magical.

But every set eventually gets torn down and all the parts mixed together into our lego-bin. I think there are probably people who only follow the instructions, but most people probably build the expected thing and then later go on to play alone.

I have a lego tattoo, so I'm biased, but I think lego is still an amazing thing, even with the very expensive niche sets. 99% of the time my child plays with it, alone, with me, or with his friends, it's very random constructions purely from the imagination.