Remember that not all metamask user is using Uniswap or other DEX that needs extreme transaction speed in case of network congestion. The reason why they might not save the gwei is because they are making sure that people will not mistake on using higher fees. If a user want to transfer fund during congestion and want the transaction faster than the default fasted. They will manually adjust the gwei but what if they will send again in normal speed but forgot to change back again the gwei in default? It will cost 20$ ~ 50$ for a normal transaction if the feature that you want implemented.