Different use cases have differing requirements for uniqueness though. A lot of stuff doesn't need "you'd have to generate 1 billion v4 UUIDs per second for 85 years to have a 50% chance of a single collision." sort of guarantee.
You might think your "Uber, but for short term giraffe rental" startup needs that sort of guarantee in the investor demo prototype, but it doesn't. Just use an auto increment in Postgres or MySQL (or an integer primary key column in SQLite). If you fool those investors into pouring the money pipe over you, your first real technical/senior engineer hire is gonna throw all the code you have running on your laptop away anyway.
Maybe someone at Meta sat down once and figured "we have 4 billion users, each of who on average has 3 cats that they take a dozen picture of every day, so about 150 billion cat pictures per day. So if we name them using uuids we're still good for almost 50 thousand years before we have a 50% chance of displaying a pic of Fluffy when we should have displayed a pic of Mr Whiskers". Then they promptly ignored the problem (or fixed Zack's code that was using php's hash("md2", $query['catname']) ).
