The correct answer is:
URLs often change.
MLA citations exclude the full URL or web address of online sources for a few key reasons:
- URLs can be very long, making citations cluttered and difficult to format/typeset.
- Crucially, URLs often change as web content gets reorganized or updated. The content may stay the same even as the URL changes.
- Citing a URL that later becomes obsolete or inactive makes it difficult for readers to find the source.
- The core identifying information (author, title, site name, etc.) tends to remain stable even when the specific page URL changes.
So MLA omits URLs to avoid citation clutter and instability over time as sites evolve. The other static source identifying details are sufficient for readers to uniquely identify and find each online work.
The key is citing information that persists rather than a URL that may not remain constant. Therefore, "URLs often change" is the main reason MLA excludes them.