> What's stopping someone from reverse engineering it transparently and making a competitor?

Mostly that Ghidra is open source and no one would be willing to go through the hassle of reverse engineering IDA when Ghidra is just sitting right there...

Decompilation isn't exactly a rocket science: just about anyone capable of hacking on clang or gcc can write a simple decompiler. The entire point of IDA was that they've done that, and also a lot of tedious, boring work on providing support for lots and lots of different CPUs. There's just no secret sauce recipe for SREs to steal - even their FLIRT tech is documented on their site.

Probably patent law, unless it’s a black-box reverse engineering, in which case you can’t use a disassembler to peek at how it works

No it doesn't, it also has a level of protection built in to stop decompiling itself

Because reverse engineered code is usually a mess, unmaintainable and takes a lot of effort to make even small improvements. Also, you run the risk of being accused of copyright infringement.