Why do airtime and data bundles expire?

I just overheard a colleague of mine taking about how data she has on her phone and how it is going to expire. Then I got me thinking and maybe someone can explain this to me.

If you pay for any of these with your money why does it expire. It's not like a food item or something that requires you to use it within a certain time frame before it expires. If I paid for it that I should decide how I want to use it if in actual fact it is not something that can expire.

Imagine you work really hard and then you get paid but then they tell you that your money will expire in a year if you don't use it. That would be terrible!

My airtime is my money that I use and unlike something that expires the network operators should not have the right to tell me how to use it. And what happens to all the expired airtime and data that people lose through this nonsense(my understanding of it now). I was thinking of refunding but that is out of the question cause of pre-paid.

But I guess this is one of those things where the government allows nonsense like this because they benefit out it in some way.