tl;dw - MBS murders anyone who opposes him, so no one will tell him it's a bad idea (Jamal Khashoggi voiced opposition to the plan).
Also, it's a bit of a stunt to attract tourists (compared to something like the Burj Khalifa) and has a bunch of promises that are just patently not viable, as opposed to an actually thought out project.
~24m into the video, the proposal for a circle is dropped in as a better alternative (albeit still crazy)