Once, all web designers had to do was to produce a creative design, wrap it in a bunch of HTML code and present it to the client.
Of course, it had to work with every web browser and still look great. No one ever had to worry about responsive web design.
Which Smartphone would you get next week, next month, next year. How big will the screen be and what will it do?
Don’t worry if you don’t know. Because neither does anyone else. We just know it’ll beat what’s around today and it’ll do stuff better. And that’s the problem with mobile development.