Indeed, and I like this set of visualizations to demonstrate that: http://zesty.ca/voting/sim
This alone is ample reason for those two parties to resist any change to the voting method. Anything else might undermine their duopoly.
So we know why the major parties would oppose voting system change. Is there something beyond the parties' word that keeps non-partisan members of the public from wanting to change voting systems?
Members of the public who want voting system change end up with members of the parties proposing superficial changes to relieve that demand that pose no or minimal challenge to the partisan duopoly, like nonpartisan redistricting (adopted in several states), California's top-two primary system, term limits (adopted in lots of states), tweaks to election scheduling, ballot access rules, etc.