Vodacaom

Why if I buy data it has an expiry date? Why can't I buy data and use it till it ran out? now I have to spend hundreds of rand every month for data that I loose because I did not use it in the time that it is valid for with. Also every time they send a sms go onto Vodaypay....... terrible, Vodapay doesn't always work plus you need data to go onto Vodapay. All the cellular services, why do you need to buy minutes when you want to phone to the same network, really it is time consuming and frustrating.