i mean these days you don't really need to buy the books to play- you can learn how to play from an Actual Play, reference the rules in a VTT, and have your DM learn from a youtuber. that really leaves WotC either in the business of making solid modules and supplements, or just making knick knacks- and they've clearly chosen the latter, at least until they can force whatever subscription model they can on the exclusive VTT they'll have for 6e
Not to mention the basic rules are just available online for free via the SRD. They don't contain any lore, flair, or images. It's strictly the rules of the game.
Most folks never did need the books to learn to play. You've always been able to learn D&D by somebody (usually their DM) explaining it. But many people end up buying books and things once they're engaged.
Like any hobby, people spend money on it when they want to. Those solid modules and supplements you mention would be a bit part of that, but also how people feel about the company they'd be supporting matters a lot (this is the so-called "reflective reaction" Don Norman writes about in his book Emotional Design) .