You're right. Nothing is a perfect method. However if someone is cheating wouldn't you want to tighten up your security measures? The idea isn't to find something that is flawless but rather something that works BETTER.
Problem is, it's not like other sports where you're a member of an association or sporting body all year round, and can be tested 'out of season'. With those - let's say Athletics, whatever, there isn't another competing organisatin you can just go and compete in. Boxing, there are hundreds. A boxer could simply 'retire', do his PED's, then 'unretire' a few weeks out from a fight. Or, there are so many different sanctioning bodies, they'd just pick any old one to sanction the fight. One with lax testing laws. Look at guys that are banned for medical reasons, they just find another location to fight, or another body to sanction the fight. You'd just end up with fights physically happening anywhere in the world that would permit it, let's say Dubai (just for arguments sake) and then broadcast into the countries with the interest in it.
of course as some of you have said, all test are flawed. and these performance enhancers are getting more sophisticated. and the producers are trying to stay one step ahead of the people who are testing, making it harder to trace. but might be possible to improve on the testing. look what happen to baseball. you saw how many guys cheated for years and got away with it until the government forced baseball to tighten thins up. not saying boxing is in that kind of situation but its just a thought.