Unique merch costs them more.
They order in smaller quantities, have to deal with the logistics of shipping and storing and eventually discounting and moving to one of their outlets for stuff that doesn't sell. When they go to refresh, they have to start the whole process over again.
Mostly the same reasons the individual resorts don't have their own uniquely designed and packaged toiletries in the rooms anymore - much cheaper to put in larger orders for generic WDW stuff than for the Contemporary, Grand Floridian, Polynesian, etc. and if they run low at one resort, they can just send a generic box of whatever there from a central storage location rather than reordering a bottle of shampoo with a unique design and scent intended for a particular location.
It's all just them cutting corners to shave pennies off their costs while raising what they charge guests for increasingly less unique and interesting products and services.
A lot of people don't care about the toiletries or the magnets or other things like that individually but it's the death by a thousand cuts principle at work. In the case of the resorts, it's not exactly difficult to draw the line from how we got from there to whole build-outs like the Riviera that with less unique and complex architecture, were easier to design, construct and will be cheaper to maintain over the years going forward.
In the case of the parks, it's easy to see why it's the same generic stuff in most places. They want to put shops everywhere but don't want to deal with the costs associated with making the merchandise those shops sell unique, any more than they think they have to anymore unless they are subletting space and making money off someone else selling their own stuff in them (i.e. Sunglass Hut, Arribas Brothers, etc.)
