To answer the question you asked, the most obvious reason you don't have that option would be because the app must not be programmed with that functionality. Why have an option for something it can't do?
Okay, I know you actually meant to ask to have the ability to schedule DVR recordings added to the app. There may be somebody in this user forum who would like to do that for you. But unfortunately, we are unable to do more than advise you to contact Cox and make your request directly to them. (You can reach them by emailing cox.help@cox.com.) A moderator may post a thank you for your suggestion and tell you it will be forwarded to their leaders. But, unless it's already in development, don't expect to see that option anytime soon. User requests don't seem to be implemented quickly.
Edit: Better than in development, GregP1 posted that functionality already exists. Maybe the app needs something that tells users about it. Does it have a help option? if it does and this functionality is included, then my revised recommendation would be the app version of RTFM.