Sterak's Gage also gives you an AD multiplier and a bunch of health. It wasn't just the shield that was the issue, it was the whole package.
The shield provided by Phantom Dancer has a longer cooldown and a shorter duration than the one provided by Sterak's Gage, and you lose out on the utility and damage a RFC or Shiv would provide in its place.
Having them be separate items allows them to be tuned separately. Sometimes, building defensive boots just doesn't cut it, and you shouldn't be forced to gut your build and scaling to get an item that will actually keep you alive.
Defensive itemization is already in the dumpster. We don't need to lose more of it.