I guess I've always looked at MHP tickets kind of like buying tickets to a Broadway show, sporting event, or other things along those lines. As a buyer, it's on you to double and triple check the dates before pushing the purchase button. Some places might allow refunds or exchanges or credits, but for the most part (in my experience), once you've bought the ticket, you've committed to it. I know some places allow you to change tickets and charge a change fee to do so, but I overall think that just complicates the issue so Disney goes the easier route and has a blanket statement saying no refunds or exchanges or transfers. I agree that tragedies do happen, so I think that's why it's always worth calling and pleading your case, but for the most part, if you can't go, that is unfortunately just the cost of having plans change.
Along with the reasons mentioned by others, I also think Disney goes the non-refundable, non-transferable route to try and avoid scalpers. Can you image how many tickets scalpers would gobble up to resell at a higher price if they could just return the ones that they can't sell? As it is, it's unfortunate to see so many tickets listed on third-party seller sites.