I have my own theories as to why I think they're popular (though I won't ever get why THIS popular) I don't use Tiktok, but I remember that before tiktok was a thing, these edits seemed to appear on Instagram first (I only noticed them around 2017-2018(?).
What always stood out to me was how it was rather focused on looking cool than anything else. Like you said before, it's not looking for any intellectual or emotional response, instead it's just visual stimulation. To have a cool vibe, basically. Not to get too carried away, but I also link it to the whole 'aesthetic-ification' of everything on the internet these past years.
I think it's also worth noting that consuming anime has changed a lot from 2000's to 2010's to now 2020's. It's also been really normalized to watch anime, so there's a lot of people new to it and searching/being recommended what to watch.
I think the typical viewer of these videos is probably more like scrolling through FYP mindlessly -> Cool anime clip of cool character doing cool thing with cool music -> discover said anime or music and like it -> save the video to remember checking it out later -> repeat cycle
I think it's partly that, and there's also people who genuinely enjoy editing it to show appreciation for certain characters, you can think of it like fancams but for fictional characters isnetad of kpop idols or celebrities.
