Why i don’t have a root privileges with suid executed?

I follow the steps:

  1. Login as root in my Ubuntu
  2. Create user alex without root privileges
  3. run chmod u+s /bin/bash
  4. Login as alex
  5. Run ls /root and got permission denied
  6. Trying /bin/bash -c "ls /root" from alex also results in permission denied

Adding suid bit to bash is equivalent to running sudo bash, with root privileges.

Why do I get permission denied?
Looks like -p bash option should help, but why?