Saying that béchamel is a fundamental thing you need to learn is quite specific to a range of European recipes and traditions. You can make lots of fantastic things, from scratch, even involving Italian food, without any mother sauces.
I would argue that the real fundamental things for basic cooking are more like how to use a knife, how to sauté things, what happens to different veggies, meats, and herbs as they cook for various times and temperatures, etc.
As for the mother sauces, they’re prepared ingredients that one might make at home to use in various recipes. There are plenty of other examples: stocks, various things that can be made out of eggs, various Asian sauces, hot sauces, various cooked garlic preparations, sofrito, etc. Béchamel is just one example.