That just feels like an extension of how things used to be done back then, especially on DOS. Making a recipe app? A bookkeeping app? A paint program? Well, you start by making a UI library first, and the product stuff got done eventually.
Then there was a transitionary period where common UIs were available, but not that great and individual shops could still easily create better experiences on their own. See eg Winamp. But eventually shared UI stacks got better and doing your own thing became a bad experience, see eg the Java UI libraries.
I don’t know about this. If you were doing a recipe app on DOS likely you were using dBase or a clone of it and all those apps looked the same because of the character based limitations of the PC. Borland environments were similar.
Macintosh was a thing at the time, and Steve Jobs was preaching desktop metaphor orthodoxy in his church. Most apps at least tried.
Yes, and for the last decade the circle has been closing. Native widgets and a common UI language are dying. Every app seems to start by collecting a hodepodge of "components" glued together half-heartedly by utility CSS classes.