What am I missing about this "guaranteed" money-making scheme that I didn't hear from anyone but just popped up in my head?
What you are describing is called Arbitrage and is done by many traders for a variety of different assets, not just cryptocurrencies. This is only guaranteed to make you money so long as there is a price difference between exchanges. The end result of many people doing arbitrage is that the exchanges all settle onto the same price.
The main difficulty in performing arbitrage is the time it takes to move assets from one exchange to another. Ideally you would buy the asset, immediately transfer it to another exchange, and then sell it. However because of physics and protocol limitations, it is not possible to immediately move Bitcoin (and most cryptocurrencies). In the time that it took to move the Bitcoin, the price may have moved to be unfavorably to you, so you may lose money, or at least, not make any.
There are ways around it such as having balances on multiple exchanges so buying on one exchange triggers a sell on another. It's not strictly the same asset, but your balance of that asset does not change. However this requires upfront capital to setup those balances.
There are efforts to make it easier to perform arbitrage by decreasing the transfer time. Layer 2 protocols such as the Lightning Network or Blockstream's Liquid network allow for fast and nearly instant transfers of Bitcoin, so they can be utilized by traders to transfer Bitcoin between exchanges to perform arbitrage.