I don't think you know what "a web thing" means. Yes the map can be run on a web server, but all the images of the chunks that are sent over the server still need to be generated and stored server side.
And yes, the actual chunk files take up more space, but that doesn't mean we can afford to also have all the image files alongside it.
Yeah, it would need 1000000000000 pixels to display the entire smp. Which would be hundreds and maybe thousands of GigaBytes of storage. Plus, my guild doesnt want the coords to our guild city leaked because yeah, its a guild only city.