The first problem is space .

The first problem is space. Most phone until recently have too little storage space to contain the entire generic kernel and android framework. A compounding issue is that phone hardware is generally more variant and less standardized than PC hardware, which can be unavoidable due to the limited space. The other problem is that there no single standard for flashing method among stock boot loaders. With PC, you have the standard booting from CD and booting from USB drive method; in the standard external drive booting method the PC is the USB Host, but Android generally acts as USB Client. This is also reflected in the choice of cables, the Android side of the USB cable is usually a USB micro and most external drives also use USB mini, this means the physical connection requires you to get a USB micro to USB micro cable, which is relatively uncommon. Finally, the last problem is political. Manufacturers and carriers have no incentives to change the status quo, only a very small minority of users are going to flash their device, which will usually void warranty. They have no incentive to encourage people to install a boot loader that makes it easy to void warranty and which often cause support nightmare.