Maybe ... because they are dogs and dog food is what they enjoy. Maybe - but has a dog ever told you he wouldn't prefer some pizza for some variety in his diet?
In the gemara in Taanis [כ] Rav Huna says that one may not serve human food to animals. Rashi offers two reasons: 1] It's a disgrace to the food that it is going to animals and not to humans. [Two big lessons - A] In Judaism we respect food! 2] Humans are higher than animals.] 2] It could be going to humans instead.
The practical difference would be if nobody will eat it anyway [such as what is left on people's plates]. According to the first reason it would still be forbidden because it is a disgrace to the food. According to the second reason it would be permitted because nobody is eating it anyway.
Bottom line - Some poskim forbid one to feed animals human food while there are those that permit this practice.
עי' שו"ת מנחת יצחק ח"ג סי' מ"ה
