Unreal is not built for computational efficiency. It's built for efficiency of content creators -- which in the AAA space is a very good thing. If you're spending $100M to make content, you don't want those people wasting half their time on bad tools and end up having to spend $200M. But in that trade-off, they give us a lot of other things to ridicule
I think the mistake people make is thinking that the Unreal Engine or Unity have been 'designed' in any coherent way at all. The scars and problems are what happens when you take an engine which was developed in the late 90s and continue to build on it for 20 years. What is cutting edge then is not cutting edge now, or indeed the right way to do it - trade offs change but when you've baked your data structures it is very hard to make that change without giving significant time to rewriting things.
(Do not believe the much repeated lie that UE4 was a complete rewrite - this is PR bullshit; segments of the engine were redone but the vast majority of the code didn't change between UE3 and UE4)
Unity suffers from much the same problem, while there is certainly a push towards better data centric designs (for example, the new job system) it requires time to swing the engine around from what was a good idea 10 years ago to how things work now.
It's rare you get a chance to gut everything and start again; I've experienced it once at Codies during the switch between Ego2 and (planned) Ego3 (Don't believe the lie which is the Ego Engine wiki page either ;) ) but it is a rare thing to do and even then was limited to graphics and resource loading for the most part, the former because Ego2 was a great XBox360 engine but kinda got worse the further you moved from that hardware