Because back compat' is a very strong feature of the language, same reason "match" was created instead of replacing switch.

As a result, taking a php 5.2 script and moving it up to 8.5 is super easy, and taking a PHP 4 one is barely harder only longer (since it probably uses the horrors that were register_globals and co).

Ultimately, I prefer this than a fragmented ecosystem impossible to resolve.