To be honest I'm not sure how true this is. I think it's more that there does seem to be quite a baked-in bias to repeat basic structures and not reuse (much less come up with) abstractions. So where that is the existing pattern it looks like it's keeping with that, when in reality it would often do that either way.
There have been many cases where I've started a piece of work by laying down very rigid abstractions and a few examples of using them, and I explicitly prompt to not only exclusively use the specific abstraction API but also copy the way I've used it. And the (frontier) LLM does neither, it just steams ahead re-implementing things from scratch from bottom up basic structures, partially and often totally ignoring the abstractions.
I don't know exactly why this should be the case but my naive suspicion is that there's just an awful lot of this type of stuff in the masses of training code and the weights just somehow 'know better' how to get results this way, rather than using your more novel abstractions/patterns.