Why does "top-up" Data expire?

Please advise:

I buy Data for my top-up account. You sell me "x" data for "x" ZAR (Rands).

Why do you offer data for a limited period only? If I buy "x" data, why can I not use the data in a period of time that suits MY needs? Why does my data expire at a certain time? I paid for the data and should be able use it as and when I need it. Why do you force your customers to use it within a time-limit?

It seems to me as if you want to cash in on your customers who do not use their data within your time limits. It seems as if you score and your customers lose out?

I would prefer that you do NOT phone or email me. I am again not interested in a general "Thank you for your feedback, we are looking into your request...."

I believe this question concerns many of your users, thus please just answer here.