Because
People are reporting issues because, duh, there are issues.
Maybe it's a design flaw that didn't show up on the more carefully assembled prototypes but does show up when they go to mass manufacture. Maybe a bad batch of components got into the mix. Maybe a batch of finished machines was built where the cheap outsourced manufacturing department and cheap outsourced quality control department both managed to be asleep at the wheel at the same time and let a bunch of lemons escape to distribution. Either way, it happened. It happens quite often.
This sort of problem child product isn't the first from Apple. Apple isn't the only company to drop a truckload of faulty gear on it's unsuspecting customers. The computer industry isn't the only one which has these sorts of problems. Big companies are just as vulnerable, maybe even more so because they're more likely to be building mass-produced items to cost in a cheap semi-skilled labour market and the numbers involved are much higher when it all goes Tiger Woods. Apple aren't immune to it just because they're Apple. ;-)
These problems are nothing new either, though in days gone by you'd take the item back for exchange or refund and maybe moan a bit to your mates at the pub about it. Now, people get online on a dozen forums, Facebook and Twitter and moan about it, and other people who had the same problem join in until there's a big echo chamber effect where a typical product problem starts to sound like the company is deliberately churning out broken hardware.
P.S. I'm sure Apple will replace your keyboard with one that has working SHIFT keys if you ask them. :)