Sim Card and Recharge Cards

I was struggling to find recharge cards for my cellphone at the Beijing Airport. There are a lot of China Mobile counters, but they only sell SIM cards but not recharge cards. I already have a SIM card so there is no point for me to get a new one. To be more correct, they HAVE recharge cards but just do NOT sell them, and I think they only sell them with a new SIM card purchase. When I asked them if they have recharge cards, they will give me lame excuses such as they sell out of it.

I couldn't quite understand this. I know selling SIM cards are much more lucrative than recharge cards, since you can set the price for SIM cards arbitrarily based on the number (i.e. a good number worths "more"), but the recharge cards are all priced the same. But why not stock more recharge cards so you can sell both of them? I am sure plenty of people would like to recharge their phones at the airport.

But now that I think of it more, the profit from SIM cards is so much more than the recharge cards that the stores want to bet on business people who want to make urgent phone calls to buy SIM cards instead of recharge cards, and they don't care about the business of selling recharge cards (is that really zero profit?). If that's the case it makes sense economically.

Maybe there is another explanation to this. If you can think of one, please let me know.