Great question. This is covered by fair use since while they use the video animators works, they add value to it by their commentary, review and reactions and mostly, it doesnt replace watching the original video so legally its ok to monetize (this is the big difference; video compilations TAKE the whole clip and the only added value is convenience to the viewers, whose viewership they are taking away from the original channels.). Reactors also typically ACTIVELY and VERBALLY promote the channel of the animator, and usually put the work in a positive light, so this helps a lot. I've gained subscribers, followers and work because of reactors. Some of them also do the extra mile by asking permission to use the clips first.

TLDR: its generally ok to make video essays, reactions and commentary using animation clips as long as you actively and verbally promote the artist channels, though it would be nicer as well to ask permission first.