MongoDB ships with horizontal sharding out-of-the-box, has idiomatic and well-maintained drivers for pretty much every language you could want (no C library re-use), is reasonably vendor-neutral and can be run locally, and the data modeling it encourages is both preferential for some people as well as pushes users to avoid patterns that don't scale very well with other models. Whether these things are important to you is a different question, but there is a lot to like that alternatives may not have answers for. If you currently or plan on spending > 10K per month on your database, I think MongoDB is one of the strongest choices out there.
Also want to add that you can definitely use MongoDB (or any other database) in a way that doesn't scale well. I have personally run MongoDB at petabyte scale and had a relatively great experience.
For most use cases, PostgreSQL is cheaper and faster to run.
A lot fewer are "web scale" than think they are. For ones who are, there are other competitors like Snowflake that work well.
As for scaling, it depends what you want to do. If you want to do things that look like joins, maybe it wasn't the right choice. Though I've definitely succeeded.
Also want to add that you can definitely use MongoDB (or any other database) in a way that doesn't scale well. I have personally run MongoDB at petabyte scale and had a relatively great experience.