It makes sense to only use a single UI toolkit. If some of your stuff is cross platform, it's probably easier to write the single-platform stuff on the cross-platform toolkit than it is to have your devs need to know 2 or more toolkits.

So developers have more transferable skills.

I had to make a web app with a friend and I used Flutter which was absolutely amazing. Easier to work in than react native, and now if I want to work on some Linux desktop apps I have the skills to do so.