In My Hero Academia, the Nomu are incredibly strong creatures that possess multiple quirks. Even a single one of them proves to be a big threat. Nomu or not, the brain of any creature is very weak and one could die if it doesn't have any kind of protection (usually skull). The Nomu seem to have brains without any kind of protection. They are clearly exposed and visible to everyone around. Considering the fact that the Nomu are strong creatures and the law doesn't restrict to kill them (since Endeavor killed many of them at the Hosu incident and also the high-end one), why don't pro heroes attack the Nomus' brains? (I know it looks gross and all the stuff, but that doesn't change the fact).
First of all, not all Nomu have their so-called brains exposed. Their description hints that they are "mindless" soldiers that need specific orders to function. Only the High-Tier ones have the capability to think for themselves. On the same note, you could wonder why they don't target the eyes of Nomu.
On one hand, giving that they all have muscle augmentation and others physical quirks, one could reason that the visible "brain" tissue is just a protective layer for the brain itself. On the other hand, it could be that the Nomu are aware of this "flaw" and one can't easily target that area, being direct attack or range attack. Also, most fights in manga/anime don't revolve around the characters targeting the weak spots of the body, meaning that even in BnHA, the Heroes don't go for the head when fighting any other villain to give them a concussion or something like that, the fights are a display of abilities and power.
And finally, my opinion is that it's just a design trait that Horikoshi - the author - liked and incorporated in the making of Nomu.