Touhou's weird in that it relies on fanworks, not official games; Osmosis, not raw sales. If we judged it by the official games alone, Reimu would've never gotten off the ground the way she did. She'd just be brushed off. But Touhou isn't the official games alone, it's also the fangames, the doujin manga, the remixes.
Think of it this way. Most games are plants, and the largest grow into towering trees. Touhou, on the other hand, is a fungus. At first glance it's just a little mushroom, but when you factor in all the different fanworks, you realize there's a massive fungal colony underneath the surface. The real problem comes from hard they are to compare, because A: The fungus is largely underground and out of sight, and B: It grows across the ground, while the trees reach for the skies. There isn't a real conversion formula here, so everyone has their own different opinions on how much the fandom matters.
Basically, being a Touhou fan doesn't necessarily mean playing the games. Even the hardcore fans that know how to use patchers are sometimes afraid of all the bullets. Still, you can appreciate the characters, the fan animations, the music remixes, etc. As a result, the mainline games aren't strictly necessary, and either way if you actually know where all the characters come from you probably know how to work a patcher too. Besides, ZUN is well aware that fan translations will come out only a couple days after release anyways, so he just lets them happen.
That, and ZUN drinks gallons of beer a day and casually decides that his OCs can eat history. He's just on a different wavelength.