This is going on the false premise that power is the one and only one reason to be a Sith. People tend to believe in the ideology they were raised in, even if it's turned every society in which it gains influence into a third world hell hole, they follow it because they believe it's right. Same is true here. Most Sith are Sith just because they were born and raised in the Empire.
It also assumes that there's only one kind of power, personal combat, that matters. Yes, a Jedi might be equally capable of strangling a guy with his mind just for laughs (just as one example), but their order forbids it. The power that might be appealing to a Sith isn't just from the capability, but the freedom to use that capability however you like. Political, economic or social power can be just as enticing as physical power.