Gut feeling? A plains taps for a single white mana. Some other cards can grant more white mana. A lot of cards can grant colorless mana of some sort, like Crystal Vein or Ashnod's Altar. White mana can be used to fulfill generic mana requirements of spells or abilities or white mana requirements. Colorless mana can only be used to fulfill generic mana requirements. The array of cards that can be used to provide colorless mana are different from the ones that can provide white mana.
To complicate things further, generic mana requirements can also be filled by colored mana of any sort.
The whole "color wheel" thing becomes involved too. A single white mana can provide a 2/1 Savannah Lions, but a blue mana won't do the same. The whole idea seems to be that certain colors of mana are more effective at accomplishing certain things.
As much as it would simplify things, there is no such easy translation system as "one colored mana=two generic mana."
If the game really did work that way, Masticore could cost WW and its abilities could cost W each.
Or Balance could cost 3! Well, Balance is broken either way, but it would be even moreso...
Or how about Sliver Queen for 2UBRG? Or Sliver Queen for 4BRG? 6RG? 8G? 10? On one hand, it's twice as much mana. On the other it can now be easily cast with a bunch of colorless mana from some artifacts or wherever. Now its synergy with Dream Halls or Coalition Victory or such things is gone. Now its not so vulnerable to creatures with protection from certain colors. Now it IS vulnerable to Dark Banishing. The list goes on and on.
I'm sure that a lot of people would be happy with a Dark Ritual that provided 2BB instead of BBB.
These examples are just scratching the surface, since this principle is so general, and so fundamentally connected to the game that it probably applies to every card ever printed (some moreso than others).
Magic is a complex game. That's part of the appeal. W is simply not the same thing as 2W. I don't need my gut to tell me that. I can use my head.
Now, I can understand where there might have been some confusion. FoWS did say that they are essentially the same and not exactly the same. The two are different. But still, "essentially the same" is an oversimplification, much the same as saying that W and U are essentially the same, when W gives you Savannah Lions and U will not, or U gives you Brainstorm where W will not.
But if you have something like Skyshroud Elf in play...