The problem is that sometimes you want a very specific format, not a locale-based format. This currently still has to be implemented in userland [1].
[1] https://github.com/js-temporal/proposal-temporal-v2/issues/5
[1] https://github.com/js-temporal/proposal-temporal-v2/issues/5
(If you are using a specific format for something other than display to a user, maybe consider the standardized ISO format instead. Machine-to-machine communications could definitely use a whole lot fewer "specific formats" and explicit Date parsing. Very few backend languages don't have out-of-the-box support or easy found library support for ISO format today.)