Because that doesn't pick up the other Canadian border crossing at the Ambassador Bridge in Detroit.
Actually I just checked the text again and "18." runs from Port Huron to Indiana on I-69, bypassing Detroit. The map doesn't show this though.
"18. Corridor from Sarnia, Ontario, Canada, through Port Huron, Michigan, southwesterly along Interstate Route 69 through Indianapolis, Indiana, through Evansville, Indiana, Memphis, Tennessee, Mississippi,..."
In Michigan, the corridor shall be from Sarnia, Ontario, Canada, southwesterly along Interstate Route 94 to the Ambassador Bridge interchange in Detroit, Michigan.