Those are Mac's guitars. 
But, to fill in the grain you use a "grain filler" or wet sand the finish (while finish is still wet)
Well, no, not exactly. We use pour filler, to be sure, but wet sanding is not to do with pour filling, and is done when the finish is a dry as you can get it, given time restrictions. Inter-coat sanding can, and even should, be done dry (though using a sandpaper that has a lubricant in the grit), and that is the part where you can fill the grain somewhat. By the time you are wet sanding, though, you need to have the finish dead flat already. It is called wet sanding, by the way, because you are lubricating the surface with soapy water, which keeps the sandpaper from loading too quickly and (among other issues) causing large scratches that you can't get rid of - the water also helps to carry away the dross from sanding.
As for
why we finish guitars with flat glossy finishes, basically because we could never sell a guitar if we didn't. The guitar buying public, by and large, expects it, and you have to provide your market with what they want. Believe me, it isn't because we enjoy it - it's a giant PITA. Bob Taylor once said, "I'd like to find the guy who decided guitars should be shinny so I can dig him up and kill him again!"
Gabriel
