I have seen this suggestion several times recently regarding GUI framework threads on HN in the past week (There has been a surge!), but they are always described in the way you did: With no explanation of why. Just an assertion that native UI kits (of which you need a separate one for each OS family) are superior; cross-platform kids are an "inferior experience" etc. I have asked at 3 times and received no answer. I say this as someone who uses both Windows and Linux regularly, and find they are functionally equivalent once you have an application launched. There are ABI differences, driver differences, file system differences, etc which need to be smoothed over with feature gates for certain application functionality, if you're using certain functionality.
I want my applications to have the required functionality, run fast, and have a good UI. I don't care if the widgets look the same as on my other applications or the ones built in to the OS; this isn't even a thing because there is so little standardization broadly. Maybe on Macs it's standardized and cross-platform kits look out of place?
The proposed approach of "Use LLMs to vibe-code a system of 3 native kits" has cons to weigh. Or making your program Windows only or w/e for a look-and-feel reason or whatever it is has cons too. IMO all our OSes are so similar, PC software should almost all be cross-platform. Exceptions are mostly drivers and driver-adjacent code.