In fact, that is not their real name anyway. The popular food item was once called "Frenched Fries" since the potatoes were frenched -- meaning to be cut into large rectangular strips or spears. Frenched cuts are larger cuts than julienned cuts, otherwise they are pretty similar。
Americans tend to slur or reduce the sound at the end of some words. Hence Frenched Fries become French Fries, much like Iced Tea became Ice Tea。