Usually any individual walkthrough is made by one person, while walkthroughs for different games are often written by different people. It all depends on who's interested in making a particular walkthrough; there's no coordinated effort.
I suppose each one being made by one person for each different one makes sense. No way one person would be able to do all of them. Especially when they might not have all the games.
Anyone who wants to contribute to a walkthrough in any capacity can. In practice we often see one person writing the initial base of the walkthrough, and then others tidying it up later, but there's certainly no requirement for that. And for older games especially, you can probably safely assume that whoever was working on it before isn't doing so anymore, so you're not stepping on any toes by picking up something from where someone previously left off.
I feel like a link to that sort of aggregation page on an individual Pokémon's page is more productive. For the original Kanto roster who've been around for thirty years, they have years upon years of merchandise available globally and regionally, and it would be far too many images to load on a single page in addition to what's already there. Not to mention how much longer it'd make the scroll.
