Quote from Rufus
They should be as rare as they are because finding one means instant infinite food. If they were more common it would be completely overpowered.
Oh, and don't forget that hostile mobs don't spawn there either so you'll never get hurt there.
Finding a mushroom biome should be one of those "OMG, I can't believe my luck!" moments, not something that happens on a semi-regular basis.
I suppose this is true. Still, I have traveled far and wide, and have yet to find one. And at this point, it seems it is so rare that those moments will never come.