I think people do it just because they've been told it's how it is done. I think it also comes down to a misconception about what pro bodybuilder's want and "the rest of us" want.
Pro's want to look unnaturally shredded JUST the day of the show. And the things they have to do to get there are NOT simple by a long shot. The idea that people bulk up and allow themselves to get too much BF only to "cut" down to 8 or 10 percent bf percent? C'mon. Most of use want to look good all year around. My wife doesn't want to hear, "sorry hon, I'm bulking" most of the year. And if I was single it'd be even worse.
Depending on how you store fat 10% is pretty damn lean for most guys. And you can gain muscle real good at about 10 to 13% bf...maybe up a little more here and there. And even for those who don't store fat evenly they will notice a change in bf distribution over time so that basically you don't even see the difference between 10 and 13 or 14%. And yet you get any fatter than than and guess what, you still won't gain more than .25 to .5 pounds of actual muscle tissue per week anyway in ideal circumstances. All these regular guys claiming 10 pounds a week and all this shit it's bullshit and I ignore cause it is the fact that they are gaing fat mostly but can't see it. Some of it is glycogen storage as well of course.
This is one of the reasons why skinny guys are always claiming to have gained unbelievable amounts of muscle in a short time. It is really easy to find examples of this.
Look at this link:
http://www.hardgainerhelper.com/
YET ANOTHER example of one of my biggest pet peeves. The guy who wants to see you a book after training a population of ONE. Himself, lol.
But look at his before and NOW pictures. The illusion is he thinks that the weight is ALL lean muscle tissue. But it's not. Notwithstanding the fact that he looks like me at 12 years old with not training guidance whatsoever
(ok, I looked much better) he sincerely thinks he gained 49 pounds of muscle. And the first 26 pounds in 89 days. No doubt he did gain 26 pounds but it wasn't all muscle.