Bethesda has been using and modifying the same engine since 1998, likely for the same reason.
The engine also has other problems like flight being just a long jump in a game which made verticality a marketing point.
If you make more or less the same game I understand that it makes sense to expand what you already have. XCOM2 build on XCOM1 engine, rather then upgrade to new unreal as devs deemed it would be u productive to mold new version of UE to their top down needs.
There is no indication that Larian would do anything different if they were building engine from the ground-up. Solasta cube based flight is cool, but is it something Larian is interested in in the first place? I haven’t seen a single reply from Larian along the lines: “that would be cool, but unfortunately our engine won’t support it”. They are throwing a lot of money into the game building new systems and rebuilding old ones - if they really wanted to do something, I am sure they would go for it.
