I will admit to doing this. Perhaps not more than four RFIs at a time but I do often post more than one at once or in quick succession.
Partly it's sheer laziness. It's much easier for me to just do them all in one go than when I finish a model. Especially since I like to photograph my models in a light box, not just on the model desk.
And partly it's because I don't post all of my work in progress builds on here anymore, but I do post all of them on RFI. If it fits in a group build or is otherwise interesting, I'll probably do a thread. There's no rush to post these as soon as I finish them.
A sterile RFI thread with ten different versions of "nice build" is pretty dull. If RFI threads didn't generate at least some discussion and interest, I don't think I would post them.
Equally, those builds aren't the only builds worthy of attention and feedback. There are a lot of models posted that I'm not particularly interested in, so i just scroll past to the next build.
Posting on Instagram is more like throwing something into the void of the Internet. It might get picked up by the algorithm and do well, but probably not. The interaction you get is shallow, it doesn't really facilitate discussion.
The format of this forum does away with the arbitrary selection of what content is shown to you. All threads are displayed equally within their own sections, sorted by most recent reply. You have the opportunity to see every thread if you just choose to click on the title if it interests you. You are in control of the content you get to see. As a result, you get much richer discussion and better interactions on here.
I've never felt there is such an unwritten rule nor that there should be. If you have something worth sharing, post it. Some people can build multiple models in a week, whereas it would take others months to finish one.
And finally, you can always choose to sort threads by date started rather than most recent reply.