Why would someone buy a 1sheeld?
Why not connect an Arduino to their smartphone using HC-06 bluetooth module and find an appropriate Android app, like for example RoboRemo. What is the advantage of using a 1sheeld?
Why would you buy a car when you can cobble something together from some old pram wheels, a lawnmower engine, some old scaffolding, gaffer tape and chewing gum?
Because buying a car is considerably easier and more reliable. Plus your "friends" don't laugh at you when they see you trying to drive it to work.
Yes, you could cobble something together that might work using random components and other bits of software, but the work involved would be considerably more than plugging in a shield and downloading an app that is guaranteed to work perfectly with that shield.
Giving an equally biased answer as @Majenko, from the other side:
Because they're lazy, they don't know anything about electronics, and they don't want to. And they have more money than they know what to do with.
I am horrified at people shelling out for an "Arduino Temperature Sensor", "Arduino Shift Register", or "Arduino LED", each of which is a generic component soldered on to a board, with pins, at a 1000% markup
I also question the claim that "cobbled together" items are more reliable, or are less likely to have people laugh at you - if you want it to look nice, put it in a nice box (this applies both to "pre-made" items, and to cobbled together).
You will also learn a lot more from making it yourself; you will learn how things work, and you have a better idea and have the opportunity to fix when things go wrong. Your projects will also be a lot smaller.
If you want something that "just works", go buy an ipad. Then, when it doesn't work, throw it in the rubbish and buy a new one. And if you want it to do something it doesn't do, tough luck.