They only all implement the ANSI SQL standard for certain values of "ANSI SQL standard." Those values being SQL-89 or perhaps SQL-99.

Beyond that it's a mess. SQL Server didn't get around to doing a really good job with window functions until this year. Last I checked, MySQL still doesn't do any number of useful things, such as WITH or FULL JOIN. Upserts and bulk inserting are different for every DBMS on which I've learned how to do them. And so on and so on. There are any number of ways you can get yourself coupled to a particular DBMS if you go beyond the core set of basic SQL features.

In other words, SQL databases are only trivially interchangeable for trivial cases.

Wrong again. Here's a very trivial example. To index or not to index? On MySQL InnoDB you usually get a major performance benefit out of indexing just about everything you want to search on later. On PostgreSQL you usually get a major performance benefit out of indexing columns only as needed later. This is because of internal design differences, sequential scans through a table are much cheaper on PostgreSQL (in MySQL it has to scan a table in key order, not physical order which means no OS prefetch). Also non-PK index scans are faster on PostgreSQL though PK index scans are slightly faster on InnoDB (basically the table is contained in the Primary Key index there).

Contrast the fairly easy and routine process of migrating between MySQL, PostgreSQL, or SQL Server with the huge amount of work involved changing your application code from MongoDB to anything else.

So suppose I want to move my database which uses PostGIS and pgcrypto to MySQL or SQL Server. How would I do that?