Curse and Leech Seed can also be passed. These don't make much sense either. Basically, Gamefreak logic.
The Japanese version of Ingrain is called Extend Roots. This is a bit different from Ingrain, which is
Firmly fix or establish (a habit, belief, or attitude) in a person.
It makes more sense if you think of Ingrain as simply extended roots that reach up from the ground, and aren't actually attached to the Pokemon. The user is sort of making roots grow around their side of the battlefield. Because which Pokemon have actual roots?