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.