"Because, much like Wii Play and 1-2 Switch they were never intended as concepts for a proper game."
Robo and Guard were very much originally intended as concepts for proper standalone games. After being shown off to the press at E3 2014, they even received spots on Nintendo's official release calendar both via PR messages and in their internal financial reports.
At the time, they were at roughly the identical stage of development as a funny little concept about human-squid hybrids shooting ink at each other, whose showfloor demo was almost the entirety of the game at that point.
Sakaguchi: The game was only about 10% complete at E3... At E3, we only had one weapon. We only had one stage, and only a mock-up of the sequence. So when we returned from E3, our big issue was figuring out how to turn all that into a product.
Nogami: At E3, we already had the heart of the game, which was “it’s fun to shoot ink”, and “it would be fun to turn that into a turf claiming game,” and we also had the play cycle of strategizing. But it wasn’t something that would keep people playing yet.
All three of those stage demos could have been expanded into full games or all three could have had a course correction and wound up relegated to miniature side diversions in other games. Personally, I think Nintendo made the right call with which one to develop into a full product and which ones to shelve and then ultimately fold into other things.