Namor's a static component of the marvel backdrop - sure, he has mood swings, but he and Atlantis are relatively stable elements that can transfer easily between narratives and stories. He's physically imposing enough as is to act as a strong, tough, doesn't-take-no-shit leader of his country and his people, but isn't so ridiculously powerful that he becomes one of the leading forces.
Guys like that - Superman, Thor, Hulk - can't be stable environmental elements, they become polarized and are always at one end or the other of major events, dying all the time and waging wars. Look what happened when Black Adam got a surge in popularity. If Namor became one of the toughest dick-swingers around, he'd lose a lot of what makes him a valuable, stable addition to stories and the general MU.
He's fine where he is.
I can only think of one time Spider-man defeate Brock without resorting to prep/trickery.. and even that one time he only slowed him down long enough to use a sonic attack. Normally Brock nosells all of Pete's attacks and he fights people in Namor's league pretty often. He can also mindcontrol/mindrape people, can become invisible, can form razor sharp tendrils (go for eyes) and I believe that in combat he's quicker than Namor. Also he himself is prety strong and hard to harm even for class 100 characters thanks to his ability to increase mass. This is not as onesided as people think.