For me it's a combination of laziness, limited time, and very limited talent. Back in the day when I played D&D, I liked to buy a mini of my character and paint it. Usually came out decent enough but took time for me to do. The thought of doing an entire army was daunting to me. So now a days I'm drawn to prepainted stuff. With any luck, my kids will get good at painting minis so I won't have to.