Two (2) very good reasons for keeping the hand on the throttle during T.O. & Climb-Out:
1) Throttles do creep out. Rare, but it does happen.
2) Training. Remember the Law of Primacy? Later, when you are flying Multi-Engine, the hand must be on the throttles to quickly snatch them back in case one fails on the T.O. roll, or after lift-off, and landing runway is still available in front. The hand must be on the throttles to effect a quick abort. If you don't do this in primary training, it will take a lifetime to instill at alater date.
But, the hand can move swiftly and purposefully to the trim control for a couple of seconds, then swiftly back to the throttle.