The US govt knows that most non resident US citizens even if they dont like paying taxes are still going to pay it because those citizens still want to be part of club US.
Just renouncing US citizenship costs several thousand dollars in legal fees and could take up to 5 years. What you are suggesting is similar to telling someone who is complaining about rent prices to just buy a house.
Well why doesn't the US government just decide to tax its citizens the same as nearly every other country in the world? You're saying that the solution to a bad system is to move, rather than just making the system less terrible.