The short story is - developers typically need to target more than one version of Java and the long lifetime of LTS releases don't line of up very well with that.

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.

Most people I know seem to use sdkman, jenv, or jabba for managing multiple jdk installs rather than rely on the system one.

Ubuntu provides Java the same way as Debian does, enabling applications to run with any compatible Java implementation. There is one version which gets pulled by default, but you can totally switch to a different one.

Because SO/Google told them in 2012 to do it that way.

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

Distros (at least Debian and Ubuntu) don't package Oracle Java AFAIK.

Oracle Java cannot be packaged or used in any but personal setup, and there’s no need for it these days when OpenJDK is the official Java implementation.