Because Jesus was a man tempted in every way just as we are and yet was without sin.
*Reason* doesn't demand that Mary be sinless. *Reason* could just as well conclude that Mary needed to be a normal sinful human being who have other children so the Jesus being raised in a normal everyday big family, by imperfect parents, would be tempted in every way just as we are, yet without sin.
How could he sympathize with our weaknesses if He never experienced them?
The whole theology of Mary being sinless is just wishful thinking on Catholic's part and has ZERO support from Scripture, where Scripture tells us that Mary called God her Savior, and it tells us that ALL have sinned and fall short of the glory of God.
Very True. Much of what God did defies our reason and God warned us not to put our reasoning above his word for that reason. “Trust in the Lord with all thine heart; and lean not unto thine own understanding.” Proverbs 3:5
Quite the OPPOSITE; in fact!
What would you call a SINLESS person who gave a SIN offering?
A hypocrite?