why isn't it called 肉菜?
This isn't the right way to think about it. Rather, there is a word 「葷菜」 which can take on the meaning of meat dishes in some contexts, if you need to translate it into English. In other contexts, 「葷菜」 may just mean dishes which have a potent smell or flavour, which is probably its original meaning.
「葷」 (Baxter-Sagart OC: /*qʰu[n]/, strong-smelling vegetable > strong-smelling dishes) is a standard semanto-phonetic character, comprised of semantic 「艸」 (vegetation) and phonetic 「軍」 (/*[k]ʷər/).
Note that /*qʰu[n]/ is also the reconstruction of 「熏」 (smoke). This indicates that 「葷」 and 「熏」 are cognate, and the original meaning of 「葷」 was something to do with a strong smell. See the Proto-Sino-Tibetan reconstructions for other cognates and modern descendants.
「葷菜」 sometimes taking on the meaning of meat dishes is just a semantic extension, likely a deliberate contrast to lighter-flavoured or blander dishes, of which vegetarian dishes (「素食」) are commonly associated with.