JavaScript: Why does replaceWith() return a string and not HTML?

Question

JavaScript

Good afternoon!
Why, in this case, does replaceWith() return a string (HTML entities) and not HTML?

document.querySelectorAll('.entry-content em').forEach(function(entry) {
  entry.replaceWith(entry.outerHTML);
  // возвращает: <em>Hello!</em>
  // а нужно: <em>Hello!</em>
});

How to force to return the working HTML code, in this particular case?
Thank you!