Flash stun and knock up + ult = gg
Flash has a 5 minute cooldown and as a summoner spell, every champion can (and usually does) also have it.
Also - Flash requirements on combos (
) means a 5 minute cooldown between each combo, which is a substantially bigger detriment than an advantage.
Especially if the combo can be flashed out of.
And if you're talking Flash combos? Riven's is frankly terrible. A solid Galio ult is terrifying and will obliterate your entire team. A solid Annie flash+Tibbers out of knowhere will obliterate your entire team.
A Riven Flash+Ult combo will kill, like, 1 squishy in your back line.
It's really hard to catch riven
Her primary defensive mechanic is mobility, not range or defense.
If she was easy to catch - her primary defensive mechanic would suck.
Annie walks as slow as an old lady and has no dashes
Annie also has 600 range, a 1.75 second team-wide stun, can build like an off-tank, doesn't need animation canceling to deliver burst, can easily kill an opponent under turret without even getting hit by a turret once, and has such crazy bases she still does tons of damage as a support.
If Annie was mobile she'd be blatently OP, she's got solid strengths essentially everwhere else.
Riven gets a shield
Annie gains a small amount of bonus armor and magic resist
Annie's shield is pretty solid, 50 armor/MR isn't exactly low: and that thorns component is brutal.
Also: riven is melee, Annie is ranged.
Riven also can't afford to purchase the same level of bulk as Annie and deliver meaningful damage.
And most light-fighters have a much better defensive mechanic than Riven, one with 100% damage-block capabilities.
Invulnerability, Invulnerability and immunity to CC, 100% evasion, spell-shields, refusal to drop below 1 HP, stealth, clones capable of eating champion-only skills, and Yasou's "LoL what ults" wind-wall.
All these fighters put Riven's shield to shame in one way or another.
All of these fighters also have meaningful power in their defensive mechanic if they're behind, where a Riven that's behind has a weak HP shield.
Riven can life steal
Annie can use a potion
Annie can also use Zhonya's. Considering what Fiddlesticks can accomplish with Zhonya's via throwing his squishy self in the middle of multiple enemies vs. what Fiddlesticks can accomplish with Spell Vamp in the same scenario (worth pointing out spell-vamp is a lot higher than life steal in items ATM), I'm going to say Zhonya's is better.
At least for the in-combat survival bonuses.
As far as drawn-out sustain goes: It's worth mentioning Riven's HP/5 is the worst in the game and none of her REWQQQR combo moves apply life-steal.