It's not about competing, there's nothing special or superior about iMessage.
It's about interoperability when you want to message someone.
If Google had it's own version of iMessage, that was exclusive to Pixel Phones, and Samsung had another version exclusive to their phones, and say, both versions were just as good, or even better, that still wouldn't solve the problem. The problem is you have a widely used messaging app that you can only use to message people who have the same phone type as yours. Whereas for all the other messaging apps, it doesn't matter squat what phone the other person has.
These companies don't want to steal iMessage, they merely want to be able to implement a messaging app that can read and write to the same standard, so that people with iPhones and people with non-iPhones can message each other without only some of the recipients being able to read the entire message content.
It blows my mind that people wouldn't want that. Do you really actually like it that you can send an iMessage to someone, and unless you happen to know for sure that they have an iphone, you then have no idea at all if they can even read the entire message or not? Is that something you genuinely think is just fine and dandy?