Unfortunately that's how the data is provided [1]. Maybe because the Great Lakes were carved by geologically recent glaciation, and the model is built off tectonic plate movement.

A writeup of the methodology is available if you're interested. [2]

[1] https://www.earthbyte.org/paleomap-paleoatlas-for-gplates/

[2] https://drive.google.com/file/d/1-q0WIa7ofISFHyBe4UxvN8DIPs8...