Because a keyboard, mouse, and large display are a lot more convenient and efficient than a small touchscreen device? Because a real computer operating system is generally more reliable than Android or iOS? Because Web sites designed for real computers render like shit on small touchscreen devices?
What are you, 12?
I only use my phone when I'm far away from a real computer. The phone exists in my life for convenience only, and only because it replaces a flashlight, an iPod, a less capable phone, and a PDA. I don't even browse OF on my phone. It's so much easier on my computer.