If you can peg BTC to USD, then why not peg all currencies to USD?
Let's have an exchange where EUR is pegged 1:1 with USD. Also, let's peg japanese yen at 1:1 with USD. Also, let's peg Zimbabwe dollars at 1:1 with USD
Actually, why stop at currencies?! Let's peg all things together at 1:1, so that $1 buys one apple which buys one laptop which buys one house! As long as you have one of something, you could have one of anything!
There is no difference between pegging houses to dollars at a 1:1 ratio vs. pegging Bitcoin to dollars at a 1:1 ratio. Both are equally absurd, and for the same reason.