But there's really no way to know that dumbed down generic merch is ALL guests want. The problem is, selling all the same stuff in nearly all the shops on property is an easy way to make spreadsheets look good because you save money in terms of production and distribution of items, but you actually never know what guests would have purchased had there been a greater variety.
The major issue is that for quite some time WDW has decided that simply making money on selling items isn't good enough. Instead of having a greater variety of items that sell in various shops (which also adds to a guest's experience in terms of theming), if the item is not a big seller, they decide not to offer it. Every single item must be highly profitable, and move lots of units.
They could make plenty of additional money off of selling a smaller number of resort-specific items, but in their minds its not worth the effort any more. Whether that is a good decision (in terms of guest loyalty, or some other intangible result) is up to interpretation. Just because Disney makes a decision does not necessarily mean its the right one, for either guests or shareholders.