Perhaps because Disney is a business and it doesn’t want to put all of its eggs in one basket?
To put it another way…
Between 2008, when Marvel Studios first launched, and now, there have been a total of 11 Marvel related properties (9 of them tentpoles) released from 4 studios. If Marvel released 3 films a year, in that same time frame, that number would increase to 15. If it released 4 films a year, that number would increase to 20. 20 films in five years! That is ridiculous, and it is absolutely understandable why any business wouldn’t want to take on that much risk.