So I was doing some Googling &
apparently, beyond the Spider-Man Franchise & his cast of characters that belong to Sony right now. The Hulk & Namor Characters / IP are owned by Universal Studios for Solo Film release rights.
So the reason you see Marvel only using "Hulk" with other characters / Marvel Movies is because of the complication due to Universal owning Hulk & Namor.
So anytime we see "Hulk & Namor" in a Marvel movie, they have to be paired with other characters in their respective movies.
We'll never get to see Disney have solo "Hulk & Namor" movies until Universal is willing to sell the rights back to Disney.
I guess we're still dealing with the fall-out of the previous person who decided to sell off Marvel Character/IP rights to various Movie studios, even today.