I'm guessing because when it comes to beefy desktops and chubbier but port friendly laptops, the Windows ecosystem beats them out on price. They don't want to get in a price war with anyone as they will drive down their famous 30-40% margins. I also think Apple is dedicated to stop being a traditional computer company entirely and to become a mobile-only entity in the long run.
I think Cook and other leadership are really focused on Jobs' vision of the post-PC period. The problem here is that the PC's never went away. We just stopped updating them as often. I'm afraid Jobs died when it was looking like PCs could ultimately be replaced by mobile devices in most circumstances. I imagine Cook is under pressure by the board to "fulfill Steve's last bullet points" which was an email that was famously leaked recently. It seemed very 2007-2010-ish level of thinking, when the mobile revolution was in full force, but come 2011-2016, suddenly tablet sales stagnated and PC sales didn't bottom out and have recently gone up.
Sadly, with VR and AR possibly being the new big thing, a beefy video-card is all but mandatory and not the kind of thing that can be shoved into a 3-5 watt SoC. That's another big thing Apple is going to ignore because it goes against its mobile-only vision.
Apple is in a strange position because no one there seems to have the political clout to say "Steve was wrong, the post-PC world isn't happening and we need to beat Lenovo, MS, and Dell on laptops, desktops, and accessories. PC's arent going away guys!" Jobs as a dead prophet is unquestionable. This is the problem of having a cult of personality instead of proper flexible leadership. Cook has become a caretaker to Jobs' vision and Jobs' vision is probably wrong. The moves they are now making are probably wrong.