That's only if the RPG hits the tank, which unless you can get pretty close it probably won't. They're useful for urban combat where a person can get close with

That's only if the RPG hits the tank, which unless you can get pretty close it probably won't. They're useful for urban combat where a person can get close without being noticed, but for an open battlefield you really need a much more advanced guided missile system which can hit targets kilometers away. And even if you are in a circumstance where you can hit a tank with an RPG, there is a pretty big difference between slowing a tank for a few minutes while it evaluates the situation and disabling a tank. Finally, while an RPG is cheap, it's still bulky - each round is about 4 kg. It's not as bulky as a missile but no one is going to carry enough rounds at one time to threaten a coordinated group of armored vehicles, at least not without sacrificing other capabilities.