We can visit digital building whenever we want without downloading it
This is why,we don't have LDD so people don't have to download it but just use it.
J J commented
It's better to have LDD installed on your computer, so you're not dependent on an online service to work. It could be down often if many people use it or be closed permanently.
I love the idea of LEGO Digital Designer Online! Since the downloadable program is obsolete nowadays, LEGO can move the program to the World Wide Web as an online community like LEGO Ideas!
Good idea. You mean something like this? https://www.mecabricks.com/en/workshop