Sure, if you are independently wealthy you can do whatever you want, including slow focused work on a topic for months.
But in Reeves case it wasn't that. It's that the slow focused work was the work he was assigned or expected to do. He didn't just do it because he was rich, he got paid for it, as part of his preparation for playing John Wick - same way actors have to bulk up, diet, learn some skills like piano playing etc before a movie.
Plus, the post's point isn't that "Reeves worked like that, why don't we all?", as if putting the onus on the individual. Yeah, "Because our boss doesn't want it, and we're not rich and can't afford to lose the job" would be the answer to that.
But Cal's point is more about "this is a good way of working, why isn't there more of it", including why it isn't being more of what employers set the working environment up for and demand (in which case you wouldn't need to be rich to be able to do it, you'd just be doing your job).
He got the job in no small part because he'd demonstrated he was able to do that in his preparation for The Matrix, his breakout role as a potential martial arts action movie star.
In that case it was more of a gamble, as the movie might have failed.
With The Matrix though, part of the film preproduction was an extended training program not just with Keanu but with Carrie-Anne Moss, Laurence Fishburne, and Hugo Weaving. Keanu was the star but the expectation was that they’d be able to train all the actors to a pretty high level. The main difference with Keanu is that you don’t necessarily want to rely on him to deliver a lot of lines; he can do it, but he’s much more of a kinesthetic, non-verbal actor at heart.
> But Cal's point is more about "this is a good way of working, why isn't there more of it"
There is some, though often half hearted. Employees are often sent to conferences with a mission to learn about "new thing X" or consultants are brought into train the team about trend Y. Encouraging structured training is a real super power. I've seen it make the different with things like "sales enablement" for GTM or effective onboarding programs for certain companies. These are oftern cargo-culted , but when down well are very impactful.
He had enough income and resources from prior movies to be able to train like this. He's also the Executive Producer of the franchise.
So this whole premise omits the key data: be rich enough to choose how you want to spend your work hours, first.
tl;dr The old adage proves true: it is expensive to be poor.