I was a Scout Leader for many, many years and rarely got thanked. It just goes with the territory.
My DC (as in District Commissioner) related a story to me which offers an explanation. Bear with me, I'll return to the point in a tick...
They'd been running some sort of competition for the District Scouts (11-14yo mixed boys and girls), and had somehow managed to mix up the results. The wrong team got awarded 1st prize.
The team realised they'd made a mistake an hour later, but it was a bit late by then. The trophy had been handed out, kids had gone home. Ho hum. Mistakes happen, right?
But one father disagreed and insisted, forcibly and volubly over the phone the next day, that his child be awarded a trophy. The DC eventually gave in, bought another trophy (with his own money), and gave it to the child in question.
The father accepted this, with thanks, and said "I understand that mistakes happen, and that most of you are volunteers". The DC, tired, fed up, and out of pocket, couldn't be bothered correcting that, "no, actually we're all volunteers, which is why someone like you making my life a lot harder than it has to be is what puts people off volunteering in the first place!!".
And that, there, is the nub of it. I think a lot of people assume there is some sort of compensation for Scout/Guide workers and "thanks" is, therefore, unnecessary since - well, why should you be thanked for doing something you're paid to do? For doing your job? To a lot of people, it simply does not make sense that someone would give up their entire weekend, and much more, sleep in a tent surrounded by other tents containing 'orrible smelly yoofs, and not get paid for it!
Only truly, deeply mad people would do that, surely? :)