Because if someone identifies outputs I own as being tainted then they can be blocked by merchants or exchanges or whatever.

"But you can just Darksend them!" is the reaction I have seen. Except that in Bitcoin you can send them to yourself and boom - new outputs. Does that make Bitcoin magically fungible? Or what if I mix the outputs using BitcoinFog, does that make Bitcoin perfectly fungible?

Fungibility is destroyed the minute you can trace transactions through the blockchain, it doesn't matter how you mix them and who is involved. Otherwise just use Bitcoin and send the funds back to yourself if they're tainted, no Dash needed!