Characters with 6s are fine because shadowrunners aren't avg. Did anyone what the NFL combines not of those guys are average, neither are navy seals, or recon marines, so it makes perfect sense to me that most shadowrunners are extraordinary physical specimens.
However having said that, it seems perfectly logical to me that, a guy who was born with 1's in str and agi would decide to cyber himself up and run the shadows, or his parents did.