I don't watch reaction videos.
Because they're insipid. People deliberately completely over-acting for clicks and views. With those thumbnails which are like
?! ?! ? !OMG PERSON *REACTS* to something actually very tame!!!!!!!!!11111
And they deliberately pad out their videos to over 10 mins so they can get more ads in and therefore more money. It's so shit.
What would have been a 2 min video if it had literally just the content - but now it's 12 mins and padded out to the extreme with 9 minutes of thanking their sponsor.
Some of them over react. But I watch a few channels that do reaction videos and for the most part I have to strongly disagree with you. One of the ones I particularly enjoy is this Opera singer (her channel is The Charismatic Voice) that has started exploring music outside of Opera. Namely Metal and Prog. She gets down in to vocal techniques and stuff like that, which I find extremely interesting. She also seems very genuine and not over-acting.
I also have no idea what you mean by ads. I've never seen a single ad in a Youtube video, unless the Youtuber has a specific sponsor for the video, which, for the channels I watch, is rare.