Browsers are using new http features much earlier than they're available in the system libraries. Browsers supported http2 and 3 before they were standardised enough to include in systems. .net http client still can't even tell you about http2 early hints as far as I understand it.

It's going to be the same for crypto and compression. Systems don't ship with brotli for example. The battle tested implementations come to the browsers first in many cases - or at least they're battle tested at the point anyone starts including them in .net or Java.