People don't need those white-label commodity cPanel or Plesk shared-hosting or VPS services to run SaaS applications anymore - it's far better for everyone[1] when they switch to a major cloud provider, namely AWS, Azure, GCP, etc.
I argue that most web/saas devs today grew up with - and tinkered-with - said white-label services: which means they got their experience with MySQL because it was part of the stock default configuration for all those web-host accounts: with PHP, maybe Perl, and many preinstalled web-applications like phpBB, Wordpress, Coppermine, phpNuke, and so on... and that's probably a solid 15+ years of commonplace web-hosting market saturation (thinking 2001 through 2016, with 2014 being the tipping-point for AWS/Az/GC being the home of SaaS).
You'd be in high-school age-range (14-18?) and get bored of simply running other peoples' programs on your web-space, and you saw these web-applications were written in PHP so you'd follow some series of online tutorials for PHP and when it inevitably leads to databases they assume you have MySQL - and MySQL (at the time) had not only the widest installbase, but also had a far more forgiving SQL engine than anyone else. While PostgreSQL was often only a few clicks away via quick installers built-in to cPanel/Plesk/etc, MySQL was a safer-bet by everyone involved.
...but those days are past: as a personal anecdote: at no-point in the past 10 years have I come across any "serious" PHP web-application software intended for private or on-prem usage since vBulletin or SVN.
[1]Except free-as-in-freedom software advocates...