Uh, because, in my experience, da sh*t don't work?

I only have experience with "slime" but I have never seen it actually seal a leak. It just makes a green gooey mess that I have to clean up.

Here's my 2+ year flat summary:
First flat: small T50 staple in the rear tire. Result: tire went totally flat, re-inflated, went flat again, had to patch tire after cleaning off green crap.
Second flat: small piece of glass, rear tire. Result, tire went flat, had to patch tire after cleaning off green crap.
Third flat: tire wearthrough, tube bulged through and got sheared off. Result: green crap explosion, the inside of my SKSs are now green, and there's a green dotted line on the road there.

I guess "True Goo" does work, judging from opinions here.

I don't generally flat anyway. I flat < 1 time a year even running the crappy Kendas my bike shipped with. I never flatted with my Nokians in the winter. I just put Bontrager Race Lite 32's on less than a week ago, we'll see how that goes.

For me, neither kevlar nor self sealers make sense. I wouldn't put Slime in my tubes now if you paid me. Tough tires make more sense to me anyway. Even if the tire self-seals, I'm going to have to fix it. If the tire is tough, I might not have to do anything other than maybe pull out a little piece of glass or something.