Canonical historically provided a single "system" version of Java (meaning any system packages that depends on Java depended on this package) which meant that that was the version of Java that you got with Ubuntu XX.XX - even for LTS releases which had a 5 year lifespan.

Ubuntu 16.04 - which is still in support came with openjdk-7 and Canonical refused to provide an openjdk-8 package for it despite Java 8 being the prevalent development platform from late 2016 onwards and despite numerous requests to do so. No one was expecting Canonical to replace openjdk-7 with openjdk-8 - they only wanted to be able to install openjdk-8 along side the existing openjdk-7 system Java.

Ubuntu 18.04 - slightly different scenario but still the same problem - openjdk-8 and openjdk-11 are available but that still doesn't cover all development use cases.

Thus, people add the PPA and install the openjdk from there because someone currently is doing the work of keeping the PPA up to date.

Suddenly the sysadmins in charge have forgotten they used this mechanism to install it on their ancient linux boxes.