Sure they created a lot of apps but they never replicated THE key feature of iMessage which is seamless SMS fallback within the same app. You don't have to even think about "Oh this person doesn't use such and such app"

Allo came the closest but messages sent to phone numbers not on Allo were sent through some proxy number so the recipient ended up getting the message as from some random google phone number.

RCS is the closest Google has ever come to the effortlessness of iMessage in terms of having a single app you can message anyone with, and incidentally there are some benefits if the other person is using a compatible app but regardless the message will get through (and from your number) one way or another.