I feel it's because (in solo q) if you don't smash early and get fed, there won't be a late game. So Stormrazor is pretty much mandatory if you want to keep up in damage compared to your opponent. Stormrazor into Zeal Item into IE is probably the best route to take to get to your late game. Riot doesn't want people to rush IE which is why they raised the price. It also doesn't make sense to get IE first when most marksmen need the attack speed, then the guaranteed crit from SR would probably out damage an IE early game, not to mention that the opponent will have that SR before you can complete your IE which puts you an item down to a very overtuned item. Late game is a theory that should work, but with the game in the state it's in, if you have to scale, then you better hope the enemy team has to scale too or that your team can help keep you safe or not feed their ass off until you reach that spike.
Concerning Jinx, it's good on her first because it gives you that extra attack speed which early levels REALLY helps Fishbones out, not to mention an SR empowered Fishbone rocket smacking the enemy marksmen in the face chunks the shit out of them. While you are right that an IE and Zeal item would be better than SR and a Zeal item, SR is better than IE early, and you need the early game dmg to be relevant. I guess it just depends on the matchups and what you feel safe doing with the team comps in the game.
It's just too good of an item and it sucks because you pretty much HAVE to buy it first on nearly every marksman just to keep up with the enemy.
