They didn't start by writing their tools for Linux, as all of these companies were founded in the eighties and started writing their software before Linux even

They didn't start by writing their tools for Linux, as all of these companies were founded in the eighties and started writing their software before Linux even existed... And Mentor Graphics initially ran on Apollo machines, which were running a proprietary though somewhat Unix-like OS (https://en.wikipedia.org/wiki/Domain/OS https://en.wikipedia.org/wiki/Domain/OS). https://en.wikipedia.org/wiki/Mentor_Graphics https://en.wikipedia.org/wiki/Mentor_Graphics https://en.wikipedia.org/wiki/Cadence_Design_Systems https://en.wikipedia.org/wiki/Cadence_Design_Systems https://en.wikipedia.org/wiki/Synopsys https://en.wikipedia.org/wiki/Synopsys