Why is bridging expensive?

Now in no means it’s outrages, but it cost me $13 and some change to bridge usdt poly to eth erc20? The transaction took awhile cause the miner/person said they wanted more money to bridge. I was using the poly beta bridge wallet.

So the question is, why cost me to bridge $13 of eth main chain to transfer $142 of usdt poly to eth main chain? After that the eth was basically zero and at the slow method took seconds to confirm.