Originally Posted by
Retro Grouch
Because you've taken a perfectly good word and changed it's meaning. Without the "aluminum" "steel" or "titanium" modifier the word "alloy" is meaningless. It definitely does not mean "aluminum" except to people who don't know what they're talking about.
Here's one definition from Google:
"a metal made by combining two or more metallic elements, esp. to give greater strength or resistance to corrosion."
So if a bike frame is made from two or more metallic elements combined into another metal, then calling that bike frame "alloy" isn't changing the definition in any way.