It's just like the code changes from a mysql connect statement and a mysqli connection statement.
There's nothing inherently wrong with the code developed for a specific version, and it *MAY* work on later versions, but in the case of PHP 7, there was a LOT of rewriting. Code developed for older versions using calls for depreciated or removed sections of PHP are going to break.
Programs written for Windows 95 may not work on Windows 10 (I'd go so far as to say MANY programs don't).
We've been doing a TON of updates for new clients to get them on board with PHP 7.
As Terry said, nothing to do with laziness of a programmer.
Yoast, one of the more popular plugins for WordPress, has started a campaign called WHIP. You can install their package on your server as an encouragement to users to upgrade - https://github.com/Yoast/whip
Yoast will also be updating their software with a big warning to users encouraging them to contact their host to upgrade, or move web hosting companies. They will then push a list of recommended hosts to move to.
if you haven't prepared for this, now is the time!