It boggles the mind that Disney encourages this type of behavior by only manufacturing limited quantities of merchandise. People want what Disney is selling, but there just isn't enough of it to go around. And given that most of it is essentially mass-produced cheap crap (looking at you, popcorn buckets and spirit jerseys!), there's no reason they couldn't just make more. It's not like these are high-end items where being part of a limited edition is part of the appeal; by their very nature, they're intended for mass consumption by large audiences.
This problem is especially frustrating (and frequent) at runDisney events, where they know the exact number of participants and their shirt sizes 6+ months in advance, yet still manage to sell out of items within minutes of opening. If it happened every now and then, it would be excusable, but it's part of the process now and only serves to encourage people to show up earlier and earlier, and buy extra merchandise to resell at a profit.
Disney loves to sell things to their visitors; they practically force it upon you when exiting through the gift shop at countless attractions and invented a whole app to sell park features that were previously included in your admission ticket. And yet they manage to consistently leave money on the table by simply not having enough product to sell.