Riven doesn't have or build any resists to back up that shield, which makes it about as tough as paper mache compared to a tank with a spammable shield like shen. Shen doesn't need to have a higher shield value because each point in his shield is worth much more than rivens due to his resistances, not to mention that anyone auto attacking him while taunted does less damage on top of that which makes his shield even stronger relatively. On top of THAT, shen has kit in built sustain, which means that shielding damage allows him to regen up his hp where as riven simply doesn't take the damage during her shield. Also shen's shield lasts longer than riven's ~1s shield. I could go on more about the shield ability itself, but you should get the idea by now.
Also, shen could easily build something like abyssal scepter for the resists, aoe mres shred, and increased shield from the ap, or any other tank item that gives ap, if he really was concerned about the raw shield numbers value he was getting, and none of this would really weaken his ability to do his job, where any time riven builds defense it's going to have a big impact on her ability to do damage - which is her primary duty.