From https://www.merriam-webster.com/dictionary/milquetoast:
Why is it milquetoast, and not milk toast?
Caspar Milquetoast is a comic strip character created in 1924 by cartoonist Harold T. Webster. Beginning a few years after the character’s debut, the term milquetoast came to describe a timid or meek person. Caspar’s last name is fitting because milk toast is a weak, bland concoction of buttered toast served in a dish of warm milk.
But it's not NOT milk toast. The English language has moved on. We Americans say 'fantastic' instead of 'fantastique' for instance.