They could, but there are also many people that just prefer native applications. I'll take Inkscape on Linux or Affinity Designer on macOS any day over a web application. They have native widgets, standard shortcuts, I can easily script them with Automator or Shortcuts, they work with the native OS accessibility options, can have integration (like drawing with Apple Pencil on an iPad on a Mac), they are more performant (can directly use native Vulkan/Metal APIs), are generally more privacy-friendly, don't consume huge amounts of memory/CPU, etc.

There is a place/market for both.