Why must data bundles expire?

I do not understand why my data bundles must expire after 30 days. I spend R149 of 1GB of data and when I haven't used all of the 1GB in 30 days, I forfeit what is remaining of the 1GB. This means that my money is gone and Vodacom is ultimately getting more money from me. Money that was paid for a portion of a product that I didn't use. This makes no sense to me because my airtime does not expire when I do not use it. It simply stays in my account for me to use as and when I need to use it and I believe the same should apply to data. It feels like Vodacom is driving the consumer to spend more money on data and crippling the consumer if he/she wants to purchase bigger products (more gigs)<br> <br> Why must data bundles expire? Why can't they be valid for longer periods as data usage varies for each consumer?<br> <br> If this can be explained to the consumer it would be appreciated. <br>