I agree it doesn't make sense, but I expect the argument is that the proposal to move to a standard gearbox cassette was ditched earlier this year. So while they struggle to produce aero bits, they feel the need to devote budget on design, testing and construction of gearboxes.
I expect the the standard cassette idea was ditched as some teams feel they have some serious secret sauce stuff going on that differentiates them from their peers. Or are doing something different in the gearbox (that wouldn't be replicated in a standard unit) that works in conjunction with the engine in a unique way.
Do I think Williams is in this category? I doubt it. So like others, it continues to look dumb to me. Clearly Williams are great at making technical decisions! The proof is in the pudding!
Richard