Simply because extensibility and plugins. Once they extend syntax greatly, plain text markdown files would contain lots of proprietary stuff no other editor would render correctly. It is also annoying to find and learn a new alternative every time a proprietary SW or service shuts down. And also some people run 100% FOSS on their machines, yes they are minority but they exist. For example to avoid stuff Adobe's updaters constantly running under root, effectively acting as a remotely acessible backdoor.