There's a pretty big stigma to just posting your IQ online. It's unverifiable so it's pointless and it can easily make you look kind of like a douche. Impressively enough that even in a place where people regularly get called subhuman bigots many are still are reluctant to do it.

I was obtuse about it but I did give a value to mine too. Plus realistically if you're talking about casual measurements not a specific testing methodology giving an upper-lower range is probably more helpful than giving a single concrete number. Even if you trust it isn't completely made up you still don't know who's listing their highest/lowest/average result.

That and there's endless debate about what constitutes a valid and accurate IQ test, so unless someone really likes to geek out about the minutiae of attempts to quantify human cognitive function they probably just want to sidestep all that noise.