If you simply put braces around it, you'd generate syntactically invalid code when you don't use braces for if-else statements.

Consider this:

  if (e)
      SWAP(a, b);
  else
     something_else();

Currently, that expands to this, which is still valid code:

  if (e)
      do {
          // ...macro...
      } while(0);
  else
      something_else();

If it were just wrapped in braces, the code would be parsed like this:

  // One-armed if-statement
  if (e) {
      // ...macro...
  }

  // Empty statement
  ;

  // Another statement
  else something_else();

It's incorrect syntax to start a statement with else, so the compiler will say something like "Unexpected 'else' at line N."

https://www.c-faq.com/ question 10.4

"What's the best way to write a multi-statement macro?"

Forces the use of a semicolon:

SWAP(…);

To scope the temporaries?