Reason is " Restaurants almost always fry their potatoes twice at two entirely different temperatures " !!!
To prepare FRENCH FRYS, we use raw potatoes.
Then,
WATER , STARCH AND HEAT CONTROL
1. Water is the enemy of crispiness
Potatoes are 80% water. In restaurants, they cut potatoes soak then blanch them.
That removes surface starch and drives out moisture before frying. If we skip this, water streams inside the oil and we end up in soggy frys !!!
2. Starch management
Restaurants often do DOUBLE FRY.
First fry at low temperature, cooks the inside of potato.
After cooling, the second fry at high temperature, dehydrates the surface fast and makes it shatter crisp.
3. Equipment and oil
In restaurants, they use deep fryers with massive oil volume and precise temperature control.
In home, it is difficult to maintain precise temperature and we END UP WITH SOGGY FRYS !!!!