Question for gold 3+ Adc mains regaurding boots

So im curious why do people tend to go for b greaves most when ninja tabi offers 12% reduction and some armour, or the merc treds offer tenacity, seems like being alive longer to get your dps off would be more beneficial. i realize that 35% attack speed is nothing to scoff at but if youre already building a bork and a zeal item is it really worth more than being able to survive a bit longer? At what point is the attack speed worth more than the utility. I mainly play as adc, jinx, varus, cait or sivir. (i dont main adc though in all fairness) and it seems to me espically on cait and jinx that the bit of attack speed seems to have less value to it than Tabi for trading with other adc's or ad midlaners that would burst me out or even ad junglers, so really at which point would b greaves be a better choice