Probably because the answer is obvious. Like all games, you can practice against the AI. Supercell doesn't pay thousands of people to sit down in their offices and hope people want to train against them. Supercell gave them random names and gives them a random deck from cards that are near your level
Although it's really improbable that Supercell would hire real human trainers, is this question on-topic? I see that the downvote when moused over says
This question does not show any research effort; it is unclear or not useful
As this question does not appear show any research, I have since downvoted the question. However, in doing some searching, I haven't found any concrete confirmation that the trainers are AI. All I have is my own personal experience that could be incorrect, but I am 99.99% sure is correct. Is the question on-topic?
Also, while this meta post may be related, it assumes that there is hard evidence, like the game's own website or a Wikipedia page. In this case, as the game is very new, there are no sources(at least, none that I could find) that confirm that the trainers are AI or human.