In my case it's because I had a shattered jaw and knocked out most of my teeth when it happened. Nothing lines-up well and it's very difficult for me to bite off pizza with my teeth without wearing it all over my face.

It's really nobody's business if someone chooses to use utensils on some things that others just use their hands.

Although I can think of one hilarious incident that resulted from using a fork and knife to eat chicken. My uncle was at a formal dinner (black tie) and they had chicken. He was using his fork and knife to be polite, and aparently the chicken wasn't real well-cooked (kinda slimy) so when he bore down with his utensils, a large piece of chicken shot across the table and went down some lady's cleavage. The Three Stooges could not have done it better.