Because it's way too boiled down, and also it's incorrect. A lot of track bikes have more trail than many road bikes. There are a few different factors at work. One is head tube angle, which affects how fast a bike's handling is. The other is trail, which is a measurement that is the combination of rake and head tube angle. Trail is how far the contact patch of the tire lags behind the steering axis. Know how a shopping cart wheel spins around to the back? it's because it has trail. The more trail, the more self-correcting or stable a bike's steering is.

Track bikes don't need to corner; road bikes do. They may need to handle fast - hence steep head tube angles - but they also need stability at speed - hence less rake to increase trail (yes kids, less rake means more trail! were you paying attention above?).

On the flip side, road bikes tend to have shallower head tube angles which are going to provide some more shock absorption and help neutralize the handling a little bit. You don't want fast handling when you're going 55mph. But before I hear jive about road bikes being made to go straight, dive into a 90 degree corner at 34mph and then talk. So they have more rake so that their trail measurement isn't too great. A little more rake can also help with absorbing some road chatter, as a lot of road forks do flex nicely.

Resources -

Dave Moulton's post on the subject of front end geometry:
http://davesbikeblog.blogspot.com/20...le-bit-of.html
Blog post with a bunch of links:
http://nooneline.blogspot.com/2009/0...ube-angle.html
Don Walker's write up of track geometry:
http://www.urbanvelo.org/issue3/urbanvelo3_p44-45.html

...and to answer the OP's question, front end geometry is only a little bit of the general differences between road and track geometry - each of which vary enough to the point where there's overlap. There are some other differences in seat tube angle, chainstay length, and then other design factors that might not strictly fall under the "geometry" category; as well as rider set-up, fit and weight distribution variables.

Don't make the mistake of thinking the steeper the better for all purposes. Steep track bikes are fun but are not the be-all and end-all.