Why dont people play Jayce as ADC?

because his attack range is only 500, below the average 550. or the trist/jinx/cait/kog gods.

putting him at a substantial disadvantage in lane.

its the same reasoning why teemo isn't seen as ADC.

http://leagueoflegends.wikia.com/wiki/Range

(you can argue sivir is an exception at 500 AA range as well, but she has engage/escape, spell shield and reliable burst)

because he does not have enough range and to do a lot of his damage he must get in melee range which is the complete opposite of what an adc should do. he just isn't an adc lol

Not enough AA range on his normal AAs. Also- when your adc goes melee form to hit that last combo for a kill then dies... your team gets mad at him

Scales more with levels than gold so playing with a support really slows down his powerspikes. Short auto attack range. Melee form is pretty useless against a duo lane.

He scales very well with levels.

He has a lower autoattack range than most ADCs.

Using half of his kit requires he be within melee-range of a target.

He COULD be played ADC, but there's just no real reason to since there's proper ADCs that do it better