Why does DIY auto tint eventually start looking crappy?

I know it is probably a Googleable question (as most are here), but I am lazy and figure someone might know, thus giving me a shortcut to the answer. A lot of DIY auto tint I see eventually starts to look crappy, bubbles, and such that cause it to look crappy. Is it just the product is not as good as what the pros use, how it is applied by the buyer, or a combination of both? Why does professionally done window tint looks better and last longer without lifting and bubbling?