I can't decide if it's a feature or a bug that there are at least 3 major type checkers for Python.
When the pain of maintaining larger codebases resulted in attempts to add static type checking (ultimately successful), it was, and still is, impossible to produce a solution that would work equally well for all kinds of codebases. Stricter tools show too many "false positives", laxer tools miss too many cases that a real static type checker would flag. Say, things that Django does (not to mention Zope) can only be described well by special-casing them. Hence different tools which strike different kinds of compromises.
Is it? Mypy is semi-official and has Guido as a contributor. It also predates both pyright and pyre by many years.
Honestly I find it somewhat sad that huge tech companies create their own instead of contributing to an existing and popular implementation. But perhaps the internal versions predated Mypy by a lot.
Pyre's reason is interesting, https://github.com/facebook/pyre-check/issues/38. Pyre is based off of Hack, facebook's php compiler. Facebook made pyre to re-use tooling/techniques from Hack on python. Secondary reason is performance. Pyre is done in OCaml and having same performance in type checker implemented within python is hard.