RHEL7/CentOS 7 is one of the more significant distributions out there, you can yum install version 1.12 of nginx. It wasn't until 1.13 shipped that nginx picked up any support for http/2.
On the Apache front, mod_http2 didn't ship until 2.4.17, again CentOS 7 and other RHEL7 based distributions lags behind on 2.4.6.
Sure, that doesn't mean you couldn't compile / install your own version, but for a lot of people that's just not likely to happen. Sticking with the distribution version keeps you within any support contracts, gets you security patches etc. and all the information you need to keep auditors and the like happy.
nginx is not shipped by RHEL so you are probably pulling from epel. you can also pull the latest stable version directly from nginx's repo http://nginx.org/en/linux_packages.html#RHEL-CentOS which has supported http2 since RHEL 7.4 when they released alpn support in openssl.
you can also install the latest version of apache from red hats software collections repo that supports http2 but it throws everything into /opt/rh/rh-httpd24/ which is a bit weird.
On the Apache front, mod_http2 didn't ship until 2.4.17, again CentOS 7 and other RHEL7 based distributions lags behind on 2.4.6.
Sure, that doesn't mean you couldn't compile / install your own version, but for a lot of people that's just not likely to happen. Sticking with the distribution version keeps you within any support contracts, gets you security patches etc. and all the information you need to keep auditors and the like happy.