Tablets, phones are expected to be bundled as they are expected to work out of the box, and were designed for that particular OS (the different model of ARM and friends in most cases).

Laptops, PCs are historically not, they are expected to be installed and customized, and were NOT designed for a particular OS, but work as "general purpose computing" (the universality of x86 in most cases).

There's no slippery slope, it's the customers want their general purpose device to be general.