There are very few ninjas you can buy outright. 1 version would be the time limited shop where you know that 10 ingots = 1 point, but that's still you spending those ingots on something else to get points in that event so I guess it's not buying them outright. Another would be Sakura missions where you know that 1 box=10 points. Sometimes they have like Nagato in Konoha gift shop, but for the most part you have to buy an rng item to get points to buy ninja like dice,Sakura food sacks, Fateful choice orbs,tree coins etc. People would probably complain if they implemented a feature where you could buy ninja outright because the prices would be just as high as event prices. They're called gacha games for a reason lol.