We may also note that around the time of the Arab conquests super-heavy cavalry was somewhat out of fashion in the Middle East. Byzantines and Sassanids alike were more into moderate armour and only partial barding (and the Sassanids apparently preferred to bow to the lance as the primary weapon, too) rather than the massively heavy juggernauts that had been all the rage a few centuries earlier.
'Course, you can kill even such "katatanks" with decent maces relatively easily enough, as the Romans' local auxiliary infantry at least once did to Palmyrene cataphracts, so eh.

Also, cavalry played a relatively minor part in the Conquest period Arab armies AFAIK. The largely arid peninsula wasn't exactly prime horse-rearing country after all.