If I declare two aws_route resources for my route table, I don't want a third route existing and being invisible.

I agree that there is no logical reason to not want a service gateway, but it doesn't mean that it should be here by default.

The same way you need to provision an Internet Gateway, you should create your services gateways by yourself. TF modules are here to make it easier.

Everything that comes by default won't appear in your TF, so it becomes invisible and the only way to know that it exists is to remember that it's here by default.