Stretchy CSS Style: Why so rare?

I'm not a pro web developer, so I imagine there are a lot of concerns that I just don't understand. One of them is why people don't use stretchy layouts for like, freakin' everything.

Seems to me this would be a cheap way to make a web site amenable to a lot of different devices, and a lot of different styles of human interaction.

My untutored sense is that spendy websites are static in height and width, almost always. Now this gives a very fixed layout of visual elements, and so the client sees exactly what they're buying. So it's like someone pitching a magazine ad.

This is another one of those matters where I'm throwing this question out there, and either someone will explain this to me in horsey-ducky language, or I'll find it myself and append it later...

The precipitating event was finding this Minima-stretchy template, which I'm perfectly delighted with...