You can use MCP servers without the need to use an extension. You can either add it in the settings.json or use the "Add custom server..." button on the agent 3-dot menu.

The extensions are just for more ease of use as they install the server as well. A one click solution.

Most likely there were no ready libraries when they started. Even now there are very few that can be called stable. And even then many of them are HTML based. So if you are going to spend that much time developing GUI from scratch you might as well make it your own library and get full control.