Why can’t I buy CAD flow as an off-the-shelf software tool like any other software?

CAD flow needs to be customized to fit different methodologies, infrastructure, tool vendors, foundries, IP providers, designs, company policies, access controls, or in some cases, tool licensing models. For example, a verification flow for UVM core base verification VS SoC-based verification could be different. RTL2GDS flow for TSMC 10nm VS Samsung 14nm, Synopsys tools VS Cadence tools, or in-house compute servers VS public could be very different.