As already linked, level 1 techs do a lot of custom kvm stuff as Wendell got annoyed at the available options. For cheaper, order from China to get random configurations as they will cover a lot not niche product areas.
As to why they aren't used much anymore, have you looked into remote control options nowadays? Remote desktop, vnc variations, parsec.
As to having only one main pc. It's in the name. Main pc. You have one. Everthing else is not main pc. I have one main pc i use for stuff, and use that one, or often my phone, to vnc into desktops or web interface into software/services the other 7 machines/million VMs/switches/routers.
Then I have one smaller monitor, basic keyboard and mouse that I can easily carry to and connect up to any machine I use for initial setup or if I really have to access a machine bios.