that would certainly make it nicer! I do not know about the limits of randomness. As long as it would be enough to avoid all the instances that will ever exist accidentally bumping into each other's content.
We already have the IDs you mention in the URLs though, right? In the post I used as the example, the author's home instance URL for the item is https://lebowski.social/post/12337
<locally-unique-number> = 12337
<originating-intance-domain> = lebowski.social
To make things really simple (at least for the end user), instead of having a UUID, you just have a way that can be easily learned to transform the URL by copy/paste by hand, and would also lend itself to programmatic manipulation:
check out [this post](/local/lebowski.social/post/12337)
I still am thinking there must be some reason why this isn't done because it seems very obvious. Either it is more difficult to implement across federation than I am guessing, or there are use cases I am not considering.