Because usual subway trains won't fit there.
Why would 30 tunnels take less space than 1 infinitely more efficient tunnel?
How would Elon stations for CARS fit in smaller spaces than those packed with humans without the automobile wrappers?
Nope, it will be 30 miles in different directions . They are not supposed to be in parallel. And as you won't be running one train in 15 minutes (or few minutes in decent underground) but a car every few seconds, throughput of 1 narrow tunnel probably will be even better than one crappy LA subway line has. Then multiply by 30.
Why do you trust Elon's involvement makes it possible to do this? He is such a genius to figure out how to load/unload cars into a tunnel like a gatling gun, but he couldn't apply the same thing to a slow moving subway car? With his genius and vision, he could have traditional trains arriving and leaving every station every 20 seconds. Miss your train? Don't worry, the next one is here in 3, 2, 1.. Bing! This could never result in half a million people packed like sardines in a hole for 20 hours while a billion dollar rescue mission is executed.