The initial reason is that you'll find strict legal regulations regarding how dark the tint on various vehicle windows might be, and I am far from familiar with all of them. A professional - somebody that applies window tint for income - would be knowledgeable about the ins and outs of any applicable rules or laws.
Secondly, my car is definitely an extension of me. It represents me wherever I am going. If I have tinting about the window, I want it to check sharp and expertly applied. I would not want it to look sloppy in any way, which I certainly fear which it would were I to try to apply it myself. That would not be the impression I would want to give when driving out - far from it, in fact!
In conclusion, I know there's no other choice for me rather than to take my car to professional - somebody who has the knowledge and skills for the job the way it should be done.