They can do better, no doubt, but it is a problem that sounds like it should be easy to solve, but becomes really rather tricky at Amazon’s scale and with their catalogue size, speed of updates to it, and general dependency on the quality of data provided by the sellers. Try defining good filter values when you have thousands of product categories with millions of products, many of which have poorly defined attributes, while all of this is subject to constant change. Quite a task.
Nothing is trivial at that size but given the immense resources and market power at their disposal, I find it more convincing that it's just not a priority because their market position is already too secure.
I heard similar excuses back in the IE6 heyday for how improving the browser was "hard". Realistically 94% market share just meant that it wasn't broken from Microsoft's perspective.
