I think you've been very unlucky, as Apple traditionally does make products to a very high spec. There have been a few models that were poor, and these are usually the subject of recalls and fixes.
A few "Friday-afternoon" machines leave the factory, sure.
Without wanting to look like I'm shifting the blame: I'd suggest that environmental conditions are probably a factor in different experiences. Heat, dust, percussive shock (when setting the laptop down); who knows what else. But, anecdotally, some people do seem to have terrible luck with hardware.
I'd perhaps try to press a case for free replacement, depending on local statutory consumer laws. Otherwise, you might be well served by taking up AppleCare.
Remember, AppleCare is essentially a bet. You're betting Apple that your Mac will fail; Apple is betting you that it won't.
