Ive heard about people who have "private" collections that no one knows about. Its an oddity to me.
Collectors on the internet are often in the minority when it comes to overall collectors. This applies to various hobbies. There are many people who may be online but they will never post, or many people who do not even have accounts. I have known collectors like this. There are even cases where there is a collector that is unknown to the local area where they are. There is one good example I can think of this, when a (now locally known, but still not online) pinball collector surfaced in my area. He has one of the larger collections in my area, I'd say top 3 or top 5. It will not be an isolated case.
Many people still live under the stigma that there is something wrong from society's view of collecting. Either the things they collect are considered child's things (toys, video games, etc) by their peers, or fall into the borderline hoarding type. You can tell by looking at people's behaviours on the whole, that most people do not speak out in public vs those that do. And this translates to online communities also.
It is difficult to determine what the percentages are for video games or arcade games, because a lot of those collections are pertaining to items that solely exist in the secondary market. There is a comparison to toy collecting that may end up being of similar percentages. The idea that Hasbro estimates that in the US, the online toy collector community only makes up 5% or less of their annual sales numbers. They have information or need to know who it is that is buying their products because they need to determine whether it is worth it to offer "collector focused" items and exclusives and whatnot. And believe-you-me, if it wasn't profitable for them to do things like that, it wouldn't be done. So on best guesses from in the fandom, if the online collectors were 5%, those who collected outside of the online communities might be 5-10% in addition. With the rest being resellers or people who are not collectors.