They have - Logic 10.x has made continual design evolutions over it's life, colour and UI changes since it's debut as version 10.x (which itself was a massive change to 8.x/9.x, which was a massive change to 7.x).
LPX and FCPX share some Apple pro apps design language, and that evolves over time as well (and will continue to evolve), the trend has been to flatter design (the first versions of LPX were very dark and very bevel-ey). A continual evolution forward is imo generally better than scrapping everything you have and doing something new, unless there are *really* good reasons to do so - which do happen, and in the case of Apple apps, most of the big design changes did have these - too deep a rathole to go into here right now though.
I think some people though think they want novelty, but then complain when things have changed, which is just human nature. Personally speaking I'm happy with a slower evolution, and it's easier to deal with, than complete rethinks which often come with some version .0 baggage (as Logic 10.0 did for sure).
And if macOS next has some design language changes in their pro app frameworks, I'm sure Logic and FCPX will take advantage of those on those systems too.
Then there are bigger UI things that aren't really about UI frameworks, but improving things like waveform drawing and so on, and I think most of us would welcome these things.