The active record pattern, where you have objects like RoR's ActiveRecord or Django's Model that directly wrap a row in a table in a database, has a nasty hidden trap that rears its head once your app is complex enough.
By tightly coupling your database backend to your application models, it all but guarantees you can't switch to another database without a major rewrite of your application.
This is probably why you don't see companies switching databases. They can't.
I think we don't see companies switching databases because it would be a pain to support the transition. There are implementation specific features that could be used that would break on a switch. Then migrating all of the data and stored procs/functions/views/etc would be a pain too as they would need to be updated for the syntax of the new database. Simple things like TOP in T-SQL do not exist in other database implementations, so that is a simple thing that would have to be fixed. Other things like CROSS APPLY in T-SQL do not (to the best of my limited knowledge) exist in other RDMS, so that would be a more complicated re-write.
LIMIT/TOP is a good example actually of the sort of thing that every ORM I've used is great at covering between databases. I haven't seen any ORMs that support queries with sophisticated JOINs, much less CROSS APPLY/CROSS JOIN LATERAL.
But I would agree that, if you're using the more complex stuff like custom functions, stored procs, triggers, etc, you're much more likely to get tripped up by that switching between databases, though ORMs mostly don't touch that kind of stuff.
Anyways, my point was that it wouldn't be ORM support or tied to ActiveRecord that would prevent a business from switching databases but a whole lot of other work that would need to go on.
select a.username, p2.*
from accounts a
join posts p2 on p2.id in (select p1.id from posts p1 where p1.account_id = a.id order by p1.score desc limit 2);
ORMs are great for smoothing over some of the rough spots between SQL and OOP. It's kind of a drag to manually hard-code all of the cascade behavior for your whole class hierarchy. Hand-rolling your own caching isn't so great either. Most languages have a lot of boilerplate around setting up SQL queries and working with the results, particularly if you want to convert the rows into objects.
Flip side, some of them have a ton of their own boilerplate and confusing setup, and can generate strange errors and poor behavior. I still haven't figured out how to use Entity Framework well, despite working with C# for years. They're good enough for like 90% of things, but the other 10%, you can get big savings writing your own SQL.
Nowadays, I think Ruby ActiveRecord is my favorite. It handles everything that's reasonable for an ORM to do without a ton of complex setup, supports some more complex stuff easily, and makes it easy to drop to SQL when you need to.
FWIW, I've never actually seen any company try to switch to a different type of database.