Why do we have to wait for carriers?

So I bought a lumia 720 in july because I had felt in love with the OS. I loved it, sure it wasn't perfect, but it suits my needs, and with great elegance.

When Nokia showed us Amber in the 925, I was anxious to get it. So I waited, constantly checking in Nokia's support page if my carrier had approved the update. But no, cero, nada.

After a while, I began to be concerned. Every big carrier, even AT&T, had already released the Amber update, except mine.

So, I decided to take matters to my own hands. I flashed my 720 with the Amber+GDR2 update, which turned out to be surprisingly easy to do, and on top of that I installed the Update 3 beta. And guess what... I haven't had ANY issues, not with my signal, speeds, stability, battery or camera.

So that makes me wonder, what the hell do carriers do that make a simple update rollout, slow. Why do they need to do that?