I think there are two elements to it. First - as said above - some people come here and ask questions as "students" when they aren't. This isn't a bad thing and asking questions on the internet is easier than phoning or physically going to a club and asking the questions in person (which can be quite intimidating). I suspect that an awful lot of the really daft questions fall into that category.
The other thing is that when I'm with my FI I'm normally pretty focussed on whatever he's teaching me at the time and so thats where my questions will sit. Generic questions on some of the finer points of - for example - air law or even on the technicalities of aircraft hire in the dim and distant future when I qualify are not at the front of my mind. Lastly FIs are like taxi's, always on the clock! Small or obscure questions are often better asked of experienced pilots for free rather than an expensive FI who has another student waiting...