First of all, Bird was a career 37.6% three point shooter so by your criteria he's an average three point shooter. Not that I would personally say that. As for Curry's 3P% it's as exactly as pokoko said. The more you shoot, the higher chance you have of shooting a lower FG% (a.k.a volume shooters). The same can be said the further you shoot from the basket. Curry does both, but his FG% and 3P% are still incredibly impressive.
As for the Kobe Bryant point, you mean to tell me that to score a lot you have to shoot a lot? Wow, I didn't know that. I thought Michael just shit baskets out of thin air. Michael during the year he averaged 37.1 PPG scored 1.33 points/FGA, whereas Kobe scored 1.30/FGA during the year he averaged 35.4 PPG. A whole 0.3 point/FGA difference. That obviously must mean Michael wasn't a good scorer either by your criteria. Again, that's not something I would say about the G.O.A.T.
Also, I don't understand the insistence on why FG%/3P% is the only way to measure how great Curry is as a shooter. DeAndre Jordan has a higher FG% then Shaq, but that doesn't mean anyone would be stupid enough to say the DeAndre was a better inside player than Shaq was.
I'm not even a Steph Curry fan, but I'm arguing for him as a shooter. You're obviously threatened by the current generation of superstars so you attempt to dimish their stats by comparing them to Michael or thinking of ridiculous ways to try and invalidate their achievements.
The funny thing is, people did the same thing with Michael Jordan in regards to Oscar Robinson. There was a contingent that would dismiss Jordan at every turn because they'd decided that Oscar Robinson would never be rivaled. They'd attached their egos to that player and took any praise of a newer player as a personal insult. That's just not very logical, especially when they pick and chose which stats mean something and which don't.
As far as field goal percentage goes, you're exactly right. Someone who shoots contested shots as part of the offense is going to get consideration over a spot shooter. Taking more shots is also generally considered to have a negative impact on field goal percentage, which is why the NBA uses a minimum attemtps cut-off point.
Actually, let's just see what Steve Kerr has to say:
“I only shot when I was wide open. These guys are better shooters than I was. They have to shoot much tougher shots. They shoot off screens, off the dribble,” Kerr said. “I just shot when I was wide open with my two feet set because that’s all I could do. I wasn’t good enough.” http://nypost.com/2015/02/06/warriors-kerr-i-couldnt-be-the-third-splash-brother/
For the record, I loved Steve Kerr. The guy played his role to perfection. He didn't have to do more than his role, though, which puts him in a different category than someone like Curry or Reggie Miller who were shooting contested three-pointers more often than not.