The guy wins every single game, and people bitch about the manner in which he wins every game. He does better than any of the other guys who came before him in the same offense with the same players, and people bitch that it's just the system and the supporting cast. He completes a lot of passes and throws a bunch of touchdowns, but apparently they weren't impressive completions and touchdowns.
You know how people here say "We could go undefeated and score 30 points a game, and some fans would STILL complain." This is literally that.
Like, I get it, the guy isn't your Tommy Touchdown archetype, 6'5" with cool sunglasses and a ton of swagger and throws the ball 60 yards with a flick of his wrist. Whatever he's doing has been working, though. It took about 2 games for opposing teams to figure out our one-dimensional QBs, so if we're still waiting for the big gotcha moment, we'll probably be waiting a while.
What Purdy has that 90% of QB prospects do not is the intangible attribute that his first instinct is usually the right one, or at least good enough. Teams waste years trying to teach that when a highly-drafted QB just doesn't "get it." Then those guys bounce around for a few more years, disappointing other teams who think THEY can fix it (we have one of these on our roster). Your 4th and 5th-rounders who don't get it wind up out of the league in a couple years because they aren't worth the trouble. This is the NFL, everyone has a good arm and decent skills, except occasionally when some team is blinded by a "freak athlete." Being able to react in a split-second and have it be the RIGHT reaction is priceless.