My guess is it's something in the "that's just not the way it's done" family... Perhaps their venue controls ticket sales and doesn't care, or they have an intermediary like Ticketmaster who doesn't care. Or maybe it's more of a logistics concern, like they just think that doing such a thing would necessitate too big a change to entry procedure, likely slowing entry down too much? Or a more general logistics concern of they're not set up in person or online to do it that way, and changing now would be more trouble than it's worth?
They have to pretend to care to keep the public from rioting, but they clearly do not.
http://www.sfgate.com/business/article/Giants-Web-site-facil...
Could there be something else stopping ticket sellers from doing the same?
edit: a word
Now, if you don't want to sit in front of your computer at 5am to buy tickets before they sell out, then the scalper is offering to do that for you for a price.