Why does Riven's ult have a 30 sec cd late game with the stats it provides?

Because you add 15 seconds to it due to its duration.

"Why does an AD Carry's only steroid have a low cooldown after she's level 16?"

Yeah, Riot, why isn't it lower earlier? I mean, aside from the long-range execute.

Because if she's building full damage, come late game during teamfights she's gonna get CCed and beat down. If she builds tanky, she needs the extra damage and range to try and do her job of disrupting a team since she has meh CC in her own kit in regards to teamfight potential. If she gets so fed that she's steamrolling an entire team, SURPRISE, that's what she's meant to do.

Don't let her hit her late game while your team is still in their early game and you'll do fine.

Because she wouldn't be Riven without being horrendously broken.

Compare Riven's ult CD to Garen's. 'But their kits are different!'. Yeah. Riven has more damage, more CC, more mobility, lower cooldowns, and more AOE.

Call me when Garen's W gives him armor/MR based on how much crit he's built.

because Riven does not build 40cdr, usually she gets 25%, or 10%.

However on the off chance she does get 40%, she basically NEEDS it to teamfight at all. She has no range, no long ranged dashes (aka no real escapes), and is fairly easy to hit.

Her "no resource" doesnt mean jack shit late game anyways, since top laners like fiora, and irelia never run out of mana late game anyways.

Because in this game meaningful cooldowns are reserved for mana using champions.