Part of the answer is that each language has a different system of sounds. The human mouth can make many sounds, but each language uses only some of them. French does have a sound like 'oo', but it's a short sound. It would be spelled 'mou' in French. To a French speaker, 'meuh' sounds more like the sound of a cow .
Languages have different sounds, and they also have different ways of spelling their sounds.
Tiny babies quickly learn the sounds of their language, but it takes a long time to learn how to write and how to spell. Learning to write means learning how sounds are related to symbols in your language. If you think about it, this is a strange thing to do. You take something that uses your mouth and your ears, and you turn it into something that uses your hands and your eyes. If you're reading this page, you already know how to do that.