The estimable L.B. Jeffries wrote an essay for PopMatters about gamer burnout. Among the people he talked to, a common theme was that games are great until they start to feel like work. This is undeniably true -- nothing saps the fun of something faster than obligation. Playing games on deadline is no fun, period, and being forced to stick with a crappy game is a special kind of hell.
On the other hand, all this misses the point that playing games is a form of work. Maybe it's a challenging action game that requires players to learn patterns and hone their reflexes. That takes practice. Maybe it's a 50+ hour RPG with a sweeping story and slow-paced, strategic combat. That takes commitment. Maybe it's a sim that is best enjoyed with hundreds of dollars of peripheral hardware. That takes investment. Fun or not, getting anything worthwhile out of most video games is going to take some work.*
(Case in point: Fallout 3. That game came at the perfect time for me. I had several weeks without anything else to play, plus holiday and vacation time that let me sink my teeth into it. I loved the game, but I don't see how you could call it anything but work. Not only was there a massive time commitment required, but the missions were the sort of endless, chasing-your-own tail tasks you do at an office if you have a capricious boss.)
So when I say that I get burned out on games because they're work, I mean that less because I'm playing them as a professional service, and more because playing them is freaking draining. The last thing I do is play games for fun. They stress me out. And that's a big part of the reason why I like them, strangely enough. When I get home after eight hours of sitting on my butt updating a spreadsheet, I need some excitement. That's something video games can provide.
The burnout, then, doesn't come from getting tired of the games. It comes from getting too much into them. It's a natural byproduct of the focus and concentration is takes to succeed. After all that work, it's nice to take a vacation sometimes.
*Hence casual games, but that's another post for another day.