Yeah mysql-implementation sucks.

And (from reading the discussions on pg-hackers) was that it's very hard to keep semantics (transactions, snapshot-isolation, indexes etc).

Different storage-engines have different pros-cons (the same with indexes gin/gist). You could have:

in-memory storage engine (ex: current unlogged tables don't replicate since they don't have wal)

compressed (something like tokudb/mx)

columnar (see monetdb vs citus)

etc

It's harder to use another database because maintance + flexibility (ex: postgresql has arrays).