Here's why:
* Megan Williams is a real person: She is not a fictional character from the My Little Pony universe.
* The show is aimed at children: While the show has a large adult fanbase, it's primarily geared towards a younger audience. Introducing real-world individuals would be out of place and potentially confusing.
* Copyright and licensing: Using a real person's likeness in a fictional work would require permission and could involve complicated legal issues.
While it's fun to imagine, a crossover like this would be very unlikely to happen.
However, it's possible that someone inspired by Megan Williams could be a character in the show, either as a pony or a human. The show has been known to draw inspiration from real-world figures and concepts.