It's probably because Java promises "write once, run anywhere". If you rely on the system timezone tables, you might have a different set of timezones available

It's probably because Java promises "write once, run anywhere". If you rely on the system timezone tables, you might have a different set of timezones available from one system to another or the rules for the same timezone might differ. And then code would behave differently on different operating systems. If instead you ship timezone tables with the Java Runtime Environment, then you can promise that (by default) the code will behave the same. It sucks that it creates extra maintenance burden (and lurking problems people may not be aware of), but that's the price you pay for decoupling.