BTC noob here, please forgive me. My understanding is that you can send from any wallet to any other wallet, right? So what's to stop me from having wallets on both Huobi and LakeBTC, buying a BTC on Huobi for $3524 then transferring it to my LakeBTC wallet and selling it for $4282?

I understand arbitrage so I'm sure there's a step I'm missing, and I know there's often a delay between the order and fulfillment, fees etc, but a 21.5% spread between Huobi and LakeBTC seems like a lot.