Because I keep a monitor on my desk as a primary when I'm home with my laptop on a stand to the side. If it's smaller than a 17 it's harder to use the laptop screen as a second monitor without blowing increasing the resolution enough that I can't fit anything useful on it.

Also, preference and I'm not a huge fan of being so loyal to one company that they dictate my preferences. Been s pretty loyal Apple customer for laptop, phone, wife's computer, parents and iMac for since about 2005-2006. Between things like reducing their product line, not updating machines, not letting me set my own DNS on my phone and making political statements via their software the luster has worn off quite a bit.