IMO this is a combination of "doesn't know any better", "works for me", and "it's easy enough, don't bother me with complicate solutions".

Which is unfortunate as the "complicate" solution is often just the right solution but if the wrong solution doesn't blow up into their faces in 99% to 90% of the use cases, many people are satisfied enough with it.

I think these reasons also play a role in PHP's ongoing popularity.

Last I checked the alternatives were either very expensive Oracle or did not have a good HA solution Postgres... MySQL/MariaDB has two good HA solutions - either behind haproxy with master-master or a VIP+keepalived with Galera cluster... The issue of encodings for most applications is minor...