the idea is that you can build a generic 'REST' client and point it to almost anything that is a 'REST API' and it will work, in the same way we can now with web browsers
a REST client will never figure out /api/1/ - which is why resources go in the path and query parameters go in as, well, query parameters :)
from the server side you are right that modern front controllers and routing can make it all the same (heck, I can route resources to different subdomains, if I want), but a proxy does not know what the front controller logic is (ie. working backwards to what part is what).