I suspect it is the same reason that Nintendo doesn't push $100+ million budgets for their flagship games even though they could - because what they are currently doing works for them. The Pokemon franchise is already the industry leader in the field. Usually, the big question the people with the money ask is "If we put more money into this project, how will it translate to even more money down the road?"
Businesses run on percentages. If they put in more money to a project, they need to get proportionately more money out of that project. Using some napkin math, roughly 100 devs working at GameFreak for four years on a mainline Pokemon game translates to roughly $48 million in budget. Let's say they doubled that budget. Would the resulting game generate double the sales or profits? Would it bring more users into other Pokemon franchise elements like merch? They need a reason to justify increased expenditure, "because we want to" doesn't count.
It's easy to want things because you would personally enjoy them more but, for the sake of evaluating feasibility, it helps to put yourself in the place of an executive who has to justify the decision financially. If you depended on those earnings to fund other game projects and keep the company healthy, spending more without sufficient return on investment doesn't make a whole lot of sense.
Got a burning question you want answered?