When you buy a phone, the hardware is yours. The on-device storage is yours. Legally, the manufacturer can't access or wipe your data as it pleases. With cloud storage, your data is much much much more vulnerable to being stolen/erased by occurrences beyond your control. Including a simple change in terms and conditions. I still haven't seen a 100% reliable cloud service. And when all your data is online in the care of someone else, 99% doesn't cut it.