prolog does this too, but it's not enough. if you don't have enough data, you could find a match that will fail in the future.

To answer OP's question, the reason people rewrite is that it's "faster" to write a new one than find what's out there

OR

I don't want an entire house when all I want is a faucet. Today if you want a faucet. you might have to do something such as house = new House(); faucet = house.getFaucet(); So you have to tear apart/copy and paste the code, if it's loosely coupled enough, you find yourself dealing with all the dependencies.

This is a question lots of people have been asking tho, checkout this talk from the creator of Erlang asking the same question https://www.youtube.com/watch?v=lKXe3HUG2l4