Two reasons, one major and one a little more subtle.
Majorly, most adult bikes won't come close to fitting in a standard, non-oversize bike shipping box if you just turn the bars and fork as you describe. What counts as oversize and how much more it costs will vary depending on where you are in the world and the carrier, but you'll need a significantly bigger box and that will generally be more expensive.
Second, if you have the fork turned in the box, only one fork end will be contacting the bottom, which will prevent the fork from being able to sit flat on the bottom of the box and will cause loads on the box to have more of a tendency to side-load the fork, the direction it's weak in. The normal way of sticking a plastic fork block in the fork ends which then sits flat against the bottom actually does a lot to keep forks from getting crushed.