josh sawyer has said their engine has the best content creation pipeline he's worked with, which is probably why they're reluctant to give it up
but surely at this point they have to be doing something in the background to move to a different one. i seriously doubt they didn't try to get space-to-surface flight working, but evidently the engine didn't let them...which is more or less the same story as every other time they've tried to break out of the mold they've carved for themselves. it always ends up a janky mess.
whenever they build out actual new mechanics for the engine, like the settlement building in fo4, or the space flight in starfield, they're always just grafted on, rather than being interwoven with existing systems.
The thing that gets me is having the interior of the ships. But that interior doesn't matter. And if you try to actually RP in your RPG you inevitably get plopped on an airless planet without a suit because of the ship fast travel mechanic. The entire section of stuff there is a complete useless doodad that could be replaced with small cutscenes or static scenes to talk with onboard crew and use upgraded ship things like research stations.