Because the mempools wouldn't even accept their transactions and add them to the list of other unconfirmed ones. Or your transaction would have a low priority, which would result in you waiting days or weeks for a confirmation. It doesn't matter what you call it (ETA, mempool, or static), you have to pay a high-enough fee.

Don't rely on what Electrum or any other software shows you as a recommendation. Learn how to check the current state yourself by going to https://mempool.space/ and https://jochen-hoenicke.de/queue/#BTC%20(default%20mempool),8h,weight before you send bitcoin.