Purchase Steam Wallet via Mobile

So I've been scouring the Internet as best I can these past few days in an attempt to find answers to the question of purchasing Steam Wallet voucher cards and anything related to adding funds to your Steam account using your mobile credit.
As it turns out, there are quite a few options out there, but many, at the end of the day, in some way require credit card usage. Most of which I found to use actual mobile credit, be it SMS or Voice rates, were for Asian markets, and some North African regions.
Still, most of these require users to send an SMS or call a specific number to get their balances topped on those accounts before being able to purchase anything further.

Given that this is the 21st century, surely there must be better ways of achieving this on your mobile.
I know for Steam they could easily implement this directly, but even 3rd party sites may offer voucher purchases.
There are many factors to one could assume detracts companies from doing this, notwithstanding the profit margins and higher purchase prices required to generate income.

Question

So I ask this question, since it's one of the more widely used mediums, why can't we purchase Steam vouchers using Play Store, App Store, and even Windows Store using your mobile operator credit, airtime, or balances? (Most likely via a 3rd party app)