Disappearing airtime

So, I recently realised that my airtime on my contract expires. So basically I pay R200 per month and receive R220 airtime per month. But this airtime, like mobile data, now expires after 2 months. How is this right, as I am the one paying for that airtime, but you are then taking something away from me that I paid for. Its so nice to be a consumer. We always get the back end of the deal. Vodacom ensures that the CEO gets paid his massive salary and also his bonus so that he van buy a new BMW every year. And I pay R220 for something that dissapears into thin air. Please explain that to me? It is really unfair. Everywhere we get **********, network service providers, sars, Internet service providers, government services. It really sucks.

I also just have to note that no where since I started my vodacom contract did they ever inform me that my airtime will now expire.