Because most people can't do basic stats.
If, for argument's sake, 10% of RMBPs suffered from any particular problem, and one million people bought them, and continued to trade them in until they got a good one, based on EXTREMELY rough statistics:
100,000 people would trade in once.
10,000 people would trade in twice.
1,000 people would trade in three times
etc...
And one poor sucker would trade in 6 times.
But considering each trade in probably becomes a refurb, and I imagine quite a few refurbs are what people who trade in end up with, you can end up with a lot of exchanges required to get a good one even when the defect rate is extremely small, if you're unlucky enough to get a bad one first.
Basically, the OP's assumption that MOST RMBPs are flawed is itself MOSTLY FLAWED. (See what I did there?)
The second point is that the vast majority of Apple's customers don't care about this issue.