The universe of developers with PHP projects to maintain is filled with people who do not share the same goals and constraints that you might enjoy.
There is an enormous amount of lousy PHP code, and more being made by clueless developers. But please do not dismiss the need to support old PHPs as being driven primarily by those reasons.
1. The PHP project itself has EOLed 5.3.3, however distros continue to support it with backported security and bug patches of their own.
2. PHP 5.3.3 (with backported security and bug patches) remains the default in CentOS 6.9, which is supported until 2020. More recent versions are not available via their repositories. Hosts would have to upgrade PHP outside of the CentOS packages and assume the maintenance burden from then on.
3. About 50% of sites running PHP are at versions less than 5.5:
https://w3techs.com/technologies/details/pl-php/5/all
https://w3techs.com/technologies/details/pl-php/all/all
4. Updating PHP is not like simply updating my Web browser. Real-world production hosts like mine are filled with various work by various developers over years. Bumping PHP further than a maintenance release would almost certainly mean unnecessarily breaking things that are hard to find, probably tricky to fix and written by people I’ve never met who are long gone.