So here's the thing... If the ADC is losing lane or repeatedly forced to back with unusual amounts of gold, in other words under 1300 or 875, et cetera, there are very few options available to spend your gold on. As Sneaky says, "It's better to spend your gold on what you can, than try to save up and fall further behind."
Kircheis Shard is an acceptable alternative damage item because it still gives reliable damage, albeit only when your Charge is (ahem) full charged. The whole Stattik Shyv line is easy to buy into and upgrade, with each component/upgrade costing from 200-600 gold.
Buying Zeal over Kircheis is generally bad, but RNG willing, has a very low chance for acceptable payoffs if and only **IF **you can crit, but does give MS, which helps.
Of course, like you already pointed out, these items are multipliers and multiplying tiny base damage numbers and relying on RNG are overall weaker than buying AD for consistent damage. The lone exceptions are champions with on-hit effects who can effectively multiply their damage numbers without having to buy AD first. Vayne is the most obvious, but other acceptable ADCs include Varus and Twitch, more or less.
Having said this, the reason you've been seeing ADCs finish Zeal items first is herd mentality. LoL is a community of sheeple and at some point this season, the misnomer that doing this is somehow "correct" was born and League's herd mentality quickly bought into it. Even despite pro players consistently buying IE and ER first in every game.
In League of Legends, universal truths are more important to the community than logic, reason, individual thought and **ACTUAL **truths. Especially amongst Rito employees, who intentionally manipulate these & League's herd mentality for their own personal agendas.
#RevertPatch413