Can someone please explain the idolatry around Keanu Reeves?

Anytime he is interviewed, there is an almost "Kurt (three chord) Cobain" type swoon, where a subset seems to cling to his every word as though he was a messiah. And his acting, while not poor, sure doesn't impress me in the same way that a Lee Marvin or a Sean Connery does. (Qualifier: I am not a big "gun-fu" fan where unrealistically,... there just happens to be a bad guy everywhere he points a gun, so I might be prejudiced in that area)

So has he perhaps built popularity in ways I might not be familiar with? Has he, for instance, been champion of causes usually popular with the younger crowd? Or perhaps an extraordinary philanthropist? Does he rescue puppies from burning buildings in his spare time? I sincerely don't get it.

I think it's a combination of several factors, one of which being his high meme potential. He hosted a highly-anticipated Cyberpunk 2077 reveal several years ago which was fertile ground for memes amongst the gaming crowd and beyond. This skyrocketed his visibility online and it became a self-replicating process.

In the internet's eyes, he's also relatively scandal-free and uncontroversial (in somewhat the same vein as Bob Ross), hence the "Wholesome 100" and "Breathtaking 100" brand of memes. Like you said, this borders on being celebrity worship which in my opinion is kind of cringe.

I think it's a combination of several factors, one of which being his high meme potential. He hosted a highly-anticipated Cyberpunk 2077 reveal several years ago which was fertile ground for memes amongst the gaming crowd and beyond. This skyrocketed his visibility online and it became a self-replicating process.
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Interesting observation. So it's the digital age's equivalent of "smoke and mirrors"?

Keanu Reeves is... not a good actor. He's someone who has managed to get good press, and has certainly donated a lot of his paychecks to children's hospitals (a good thing). I strongly suspect his following has been boosted artificially, which is possible when Hollywood pays you big buck$.