It really isn't. It'd be trivial to allow mobile data to always supplement wifi data. There are apps that do this already ([1] and [2]), and some phones have the option as well (either stock or as a ROM). The problem are cellular data caps, nobody wants to blow their caps by using cellular when they could be using wifi only. There is no way for wifi to know when you're about to lose coverage, or have spotty coverage; the only way around it is to always have the mobile antenna connected to towers so it can take over. But that costs money, many mobile phone companies even disable these options on their phones to reduce congestion in their towers ([3], though it may be out of date now).
One day we might solve this problem, if we're ever able to pressure phone companies into reinvesting into their infrastructure instead of their investors.
One day we might solve this problem, if we're ever able to pressure phone companies into reinvesting into their infrastructure instead of their investors.
[1]: http://speedify.com/mobile-vpn
[2]: https://play.google.com/store/apps/details?id=it.opbyte.supe...
[3]: http://forums.androidcentral.com/samsung-galaxy-s5/402722-ho...