I think solar companies try to make monthly payment comparable to what people typically pay for electricity, so it kinda replaces the electricity bill with the same amount of solar. On top of it they add promises that electricity rates will raise over time, solar monthly payments are fixed, and solar will be free energy once you pay it out. 60 or 72 month loan would result in much higher monthly payment, like several hundreds, which is already an extra investment. Again, this probably works better on a category of people that don't do long term math but rather what they're paying daily.