Because Motor Driver IC's require the use of additional circuit components, such as caps resistor possibly inductors, which you have to mount on a PCB and connect. A fully furnished ESC Is power in. signal in, motor out, plug and play. The time you spend learning out to properly utilize the IC and understand how it works and optimize it is time you could be spending simply using a finished product. The cost is obviously higher in finances, but the cost in time and learning is something else. IF all you want to DO is make robots you don't need to know electronics at ALL. You just need to know how to put the right off the shelf components together, granted their's some way better things you can do with experience in electronics it's up to you to weight the pros and cons and the amount of time and energy you have to invest and make sure you invest it properly, or you'll get no practical returns.