In Bash, Why `then` is needed in the conditional constructs?

The conditional construct of if command looks like this:

      if TEST-COMMANDS; then
        CONSEQUENT-COMMANDS;
      [elif MORE-TEST-COMMANDS; then
        MORE-CONSEQUENTS;]
      [else ALTERNATE-CONSEQUENTS;]
      fi

And the loop construct of while command looks like this:

      while TEST-COMMANDS; do CONSEQUENT-COMMANDS; done

I was wondering why then is needed in if command but not in while command? Why couldn't it be ommited?