Whilst manufacturing tolerances, etc have all improved, so has the ability to design items to operate right "on the limit". What car makers tender out components, every cent is critical to them. So it is a double edged sword.
It is a bit like being in the military - "I go off to battle, with the best weapons that the lowest bidder could provide".
I agree that cars are more reliable, but anything that moves, turns or switches will fail. I had a Toyota Prado that had a "sealed for life" automatic transmission - you supposedly never had to change the oil. When towing the car, it was in and out of torque converter all of the time, which effectively "shears" the oil. Longer oil change intervals, higher compression and ignition temps/advance to meet emissions, cheap castings (Porsche 911 water cooled engines anyone), etc.
There are lots of stories about many cars all having problems. Jump on any bulletin board to read the tales of woe.
So, I agree that cars tend to be reliable during their warranty period, but at around the 10 year mark, better get your wallet out, things are going to start to break.