Also, I think valves strategy is to make their games hyped up by having huge gaps between the releases so when it's released people go hyper over it.
Also doesn't help that Valve has made some of the best games ever and is pretty much the only company to have little (if any) screw ups or flops. I'd say maybe Ricochet was their "screw up" but that was in their early days. Pretty much any game they've made in the past 10+ years hasn't just been good, but exceptional and masterful.
But yeah, the "long waits" also factor in a lot. Also Valve are notorious for teasing and having an incredibly elaborate marketing. Like people had to decipher code to find out that there was a Portal sequel.
Although I do find it funny that they can make Left 4 Dead 2 in literally a year but there's not a single piece shown for Half Life 3. But Valve is Valve, you can't hate 'em.