Who says?
One good upgrade to a computer -- video card, nine times out of 10 -- and you're fine for three years.
I mean, really. This is why I get ticked when people complain about PC games. I'm a HUGE PC gamer. It's NOT something you need to spend major money into every year.
Just get a GOOD system to START with -- get a top of the line video card, a very good processor, two 512k memory chips, a 19 inch or larger monitor, and a 5.1 speaker system. And a large hard drive and a good CD drive.
Yes, that will run you about $1000-1500.
Once you HAVE that, though, you're fine for three years, when it's time to replace the video card and maybe the processor.
Us serious PC gamers know this.