Good question! You are right in that when I think of all the available outlets that I could choose to print at they are still in standard paper sizes either set by a film legacy or the print industry. My work background is in print and so I am used to dealing in standard sizes and accommodating for it. Essentially it all comes down to price and what is available, at the moment the cheapest thing to do is continue to use standard sizes as machinery and supply are geared to that. As film becomes a thing of the past (especially with a large manufacturer like Kodak slowly culling film production), there will be changes I'm sure to the way that we are able to output images. At the moment a good guillotine and wasting some white paper is the easiest way to get non-standard prints. The hard part of this is that there's less printing occuring because of the digital age too so the pricing of things becomes tricky again. The printers and materials aren't cheap so businesses want to know they'll actually make money out of their printers and not a loss, hardly appealing with a dwindling market.