Why can't we carry over unused data? We paid for it.

I received a warning that I was about to go over my data limit and encouraged to either buy a block for $50 or upgrade my mobile plan. Either way, it was going to cost me more money. Then I looked at my previous two months' consumption  and  found I had only used 65% in one month and 75% the other, for a total of 1.4gb of unused data.

My question to the community is: please explain to me why can't we carry over data that we have paid handsomely for, but didn't use?  Thank you.