I'm sure there are certain users who've moved on, but I think a number of factors contribute. For starters, many of us who've helped a lot in the past end up answering the same questions repeatedly when they're answers marked clearly in the manual. There's also the phenomenon of people somehow feeling entitled and pushy about asking for help. "Make it for me" and "you're not doing it for me fast enough" are attitudes that have turned me off from checking the how do I threads quite so often.
I, personally, don't respond to as much stuff because of the ever-present flood of mobile-focused developers. I don't care about mobile development, mobile ad networks, or really anything about it, so that things out a percentage of threads I'll bother reading. I'm sure some others feel the same way.
And, really, any time spent checking forums is time not spent working on my game. Those times and moments are useful and, often, required (you HAVE to take breaks!). But I'm trying to reach certain development milestones for myself and the only way any of the 'bigger' C2 games look and play so nice is the hard work put into them.