Nintendo has been hiring new graduates to be part of their development teams so they're kinda doing what you're saying in the last sentence of your first paragraph.
As far as developing teams, it would take months (if not years) before they could even start up, let alone develop a game. Nintendo is already managing multiple teams within EPD and teams such as iQue, Retro Studios, NDCube, and Monolith Soft, let alone working with developers such as Intelligent Systems, HAL, Game Freak, Good Feel, Grezzo, Next Level Games, Camelot, Genius Sonority, and Sora, Ltd. There are other teams and studios Nintendo has or are working with that we don't even mention much because they either are working on smaller titles or are mostly support teams.
I don't know what's the mindset of Nintendo in regards on what they spend and how much they spend on projects. But they seem to have a grasp on what they need or not need at this time. If they feel that a new in-house team will not be financially worth it, then that's their stance.
Edit: Not to mention Nintendo is continuously working with third party developers to develop new games. Astral Chain from Platinum Games is an example, as well as the recently announced Hyrule Warriors: Age of Calamity from Koei Tecmo. Koei Tecmo even developed Fire Emblem Three Houses alongside some of Intelligent Systems' team members.
I know, Nintendo has its hand in a lot of projects but still, personally, I wish they would build more in-house teams. But maybe Nintendo is very wise: They probably know that inflating the market with two many games (1st and 3rd party) would devaluate the worth of each individual game. For example, if Nintendo would bring out a new Action-Adventure on the quality level of a Zelda game each quarter, it would be fantastic initially but after a while very boring, even if the games would be very different from each other, the seen it, been there-mood would eventually would catch up and an overall lack of excitement would set in.
During the NES days, Nintendo was very draconic re how many games each 3rd party developer could publish each year. Nintendo got very criticised for that practice but Nintendo knew that the games industry crash in the US happened partly because the market got flooded with trash games. I think they did the right thing. Interestingly, what we observe now is that the indy-market is flooded with too many trash games. It think it's time for the platform holders to have higher requirements for quality in their respective e-shops before a game can be released.