> How can this be unexpected?

If you routinely work with junior developers this will come up because UUID4 seems miraculous at first. There's always a bit of surprise when the question gets asked about why we have so many sequential keys when we could just use UUID4...

The amount of people with actual experience scaling databases is very small compared to the amount of e.g. Backend Engineers that end up implementing the solutions.

I think a root cause is that there’s almost never a case for preferring UUIDv4 over ULID/UUIDv7, but we still see almost all learning material reach for the former. So people only find out once they have a table that exhibits weird/low performance and suddenly need to google for specific performance problems they’ve never encountered before.

Similarly, efficiently using indexes is also a very common thing I see people not know much about, since with SQL “everything performs well until it doesn’t” (i.e. you reach scale) :)

They suggest to use UUID v7 at the end for more sequential keys. Maybe that's the new part since v7 is relatively recent.