See, there are 500gig full-size SD cards....but for some reason no one puts those in mobile, not even tablets. I mean, who ever sat around looking at a full-size SD card and said "but, darn, you know it's just TOO big".
The no sd support used to be a cost thing (save like $10 not adding that component), but now it's a profit thing where these guys are charging more than double what a 32, 64 or 128gig card goes for retail. THAT IS GOUGING. A word that gets thrown around these forums a lot, that is actually a real example.
On a related note, if you follow ultrabooks much, then you know storage has kind of been going the opposite direction due to cost of SSD's - 128gigs just isn't enough and you have to find alternatives. Same type deal where you want the OS, programs and their data on the SSD but you might store music, video and other content on an sd card.