I'm not sure whether laptops outsell desktops but it is still the case that hard disks for laptops carry a premium over the larger profile models. I don't keep up with the component choices of laptops these days, but it used to be the case that you needed the Kingston memory wizard, or whatever it was called, to find suitable memory for a laptop (if you were even able to upgrade the memory), whereas you just used standardised memory for a desktop. And so on. Not all "expansion" is plugging in external peripherals.
Of course, one might not care: I haven't upgraded the memory on my computer, but I've seen people do it to get some more useful life out of their computers instead of just buying a new one, which they may not want to do because of it being an unnecessary expense, it being a chore to migrate to another machine, and for many other reasons.
And with regard to the effort needed to make desktop products, because there has been a lot of standardisation, the threshold for new entrants is a lot lower than for mobile devices. That doesn't necessarily hurt well-resourced companies - they can finance the scale needed to make money - but you can see that people wanting to experiment with hardware and get into the game are hurt by the need to do so much more, as well as by the intensified constraints on hardware design that occur for physically smaller, lower power consumption products.