That's because for the vast majority of the time, it just isn't true - it's more a fantasy born of those who can't/won't spend more money to obtain a higher quality instrument... So, they attempt to convince *themselves* that their cheaper basses are "as good or better" than their more expensive counterparts, with that tired old line of BS...
The truth is, spending more money will nearly always get you a better instrument - although the law of diminishing returns comes into play, as the *gains per dollar* will generally get smaller as you go up the ladder... I say, buy the best instrument you can comfortably afford, then enjoy it - and don't worry about it any more...
Best of luck in your search,
- georgestrings