It'd be another way to monopolise another market, but so far weed is still federally illegal, and they don't want to deal with federal risks unless they buy everyone and legalise it :D Hmm that would be fun, wouldn't it?
But jokes aside yes they'd be money for it, cause this whole legal weed industry has a lot of problems so far, and one of the most evident is lack of stabilised varieties, which they'd have to develop first... it's chaos really in terms of commercially viable cannabis genetics, which is owed to many years of clandestine breeding.
We have very few landrace/heirloom varieties left and a lot of polyhybrids that produce 10 different genotypes. Sure they'd have the funding to get ahead, but there's another problem, they'd have to change the name, cause NOBODY WOULD BUY MONSANTO WEED and even then they'd face the risk of a watchdog from a cannabis community, that would point to rebranding efforts.
It's not impossible for them to pursue this in the future, but so far I'd doubt they see it as a viable investment. And they have sth very serious going on, which they have to bring home if they want to make their shareholders happy: