Couple of major companies like Adobe, Apple, Google, Mozilla use it in their products. That would add up quick.

https://www.sqlite.org/famous.html

Chrome's use of SQLite for history, cookies, and local storage would probably put it in first place on its own. Its extensive use in iOS and Android just guarantees the "win".

I think Sqlite's accessibility and ease of use definitely help. However, the fact you can distribute an entire db with a single file is something that shouldn't be overlooked, nor sqlite's legendary reliability and insane test coverage. Finally the fact you can embed it in memory makes for an amazing testing story.

It's also the default database used when creating new projects using many web frameworks, like Rails or (IIRC) Django.

I'm trying to understand this sentence: it seems to suggest that PHP is only popular because of Wordpress, which just... makes no sense? Wordpress uses PHP because PHP existed, and was good enough to build Wordpress on. And it's certainly not just clinging onto life, hanging around just because Wordpress is built on top of it, today?

So it's hard to understand which similarity you're asking about, but SQLite is popular because it does one thing, and does that one thing really well, and that thing also happens to be a perfect fit for complex file formats.

Kind of, at least in regards to mobile.

When developing iOS and Android apps, the "default" frameworks for storing data (Core Data for iOS, Room for Android) are wrappers around sqlite.

I would guess >50% of apps installed on your phone uses sqlite.