Can I just use ground chicken thighs?

If all you want is a chicken burger, then yes- you can use just ground chicken thighs, or even pre-ground chicken. However, the attraction for these involves the chopped up bits of skin that will bring flavor and fat, and give you crispy bits to enjoy as well. You won't get that with ground chicken meat because the butcher at the store doesn't want to bother with it either. The pre-ground chicken is also finer ground, so more difficult to handle in burger form. While I can empathize with wanting to skin the freeze/chopping, at least on a regular basis- every now and then the end result is worth the effort.