Why aren't all champions available to train with in the Practice Tool?

For the past few weeks, whenever the free to play champion rotation occurs, I go into the Practice Tool and play around with each of the champions that I'm unfamiliar with. This helps me get a better feel and understanding of their abilities, cooldowns, ability range, and spell indicators.

The fact that I have to wait weeks on end to better understand how the champions that my allies and enemies are playing work is very frustrating and I don't see any reason why a training mode against a single bot should be restricted in this way.