The reason for their quasi-monopoly is the one stated by another poster, that the 3 big EDA companies have privileged relationships with TSMC and the other major foundries so they have access to information that is not available to anyone who would want to develop alternative EDA tools for state-of-the-art CMOS processes and not for ancient processes down to "28-nm" (i.e. at the level reached in 2011, 15 years ago).
We will eventually reach a point where shrinking transistors provides diminishing returns, which should give open-source tooling and processes more time to close the gap. But we're probably still at least a decade away from seeing how that plays out, and who knows what happens with quantum computing, new materials, 3D integration, or some other semiconductor breakthrough in the meantime.
Even then, I don't think open source necessarily catches the absolute state of the art. Unless a startup fab decides to build its own EDA stack and open-source it as a way to attract customers, there will probably always be some gap between open and commercial tooling. The question is whether that gap eventually stops mattering for most designs
An excellent insight -- good tooling could be a differentiator for a US fab looking to compete on something besides price.