While you can hunt with an AR-15, it’s not the best rifle for the task.
It’s not the best rifle for any task. But it’s a good enough rifle for most tasks, and between real AR-15s and the various clones they are cheap, in common calibers, and have accessories widely available.
Which is why it’s the most common rifle in the US by a fair margin.
It being the most common rifle in the US by a fair margin is in turn why it’s so often used in public mass shootings, as those are usually done with weapons of convenience rather than something bought for purpose. Likely also why the guy who shot Trump used one.
If a public mass shooter wanted the best gun for the job, they’d get something closer to a PS-90 (the civilian version of the P-90 which is a military rifle designed for urban combat).