Card details not safe

I use VC Pay to create a prepaid credit card and link to Uber for rides. These digital cards are used ONLY on Uber and entered into the app once under payments. They are digital cards, and cannot be physically swiped. The also use Mastercard Securecode. Uber saves the card details without mastercard Securecode verification. Daily my Uber cards are being hit with ********** transactions, for example: "Your transaction for R771.49 @ BNKBILLREPS.COM ***GBR has been declined as it is for a larger value than the remaining available spend limit on the card *3338..." The ONLY place that these credit card details are entered is Uber. Please can someone explain to me why my credit card details are not safe with Uber?