data and airtime

I can't uphill today understand why when I buy data bundles and air does it expire it is just so frustrating when I buy stuff and don't use it, it expires how can I pay for something with my hard earn cash but it does not belong to me it makes absolutely no sense to me what so ever. It like me going to a shop paying the full amount for something that's going to be taken from me a month from purchase, I've been a customer of vodacom for a very long time and having doubts about hanging around.