I suspect the main reason people do it is because progressive loading has the potential to leave your page half-rendered, which looks bad. So you block the whole thing until you can present it in its entirety.

But it's the web. We're used to pages half-loading, and we're used to starting to read text before everything has been loaded. The right way to fix that problem is to make your design more lean, not to stick everything behind a 15-30s loading spinner.