Dr Who, try this thought experiment with Villages and Smithies: try to imagine a situation where you draw your hand of 5 cards, look at it, and say "Damn, I bought too many of these, I wish some had been money instead!"
Pretty easy, right? If you draw more villages than you have useful actions, well, some of those villages would have been better off as silvers, they did nothing useful for you. If you draw a bunch of smithies, that's even worse, some of them are deadweight. You have to be careful about your balance of villages and smithies, and have to decide whether maintaining that balance is worth it at all or whether it's just better to go pure-money with 1-2 smithies.
Now, try and imagine the same situation with lab. Pretend you've drawn your hand of 5 cards, and you see TOO MANY LABS! Can you imagine actually thinking that in a real game? ...not so easy then, is it. I mean, it'll happen, we can certainly come up with situations like that, but they're exceptions rather than the rule, and in most cases you're happy to see an extra lab in your hand. THAT's why lab costs 5. It would be such a no-brainer to pick up lots and lots of them if they were $4, they have to be in competition with some pretty darn good cards for there to be any hesitation at all about picking them up.