When we add vegetables to hot water (boiling), it boils the vegetables, but when we add vegetables to hot oil (boiling) it fries the vegetables. Why?
The maximum temperature for water boiling is around 100 Celsius and there is a great deal of water present. Oil can reach greater temperature and the absence of water allows other reactions, such as the Maillard reaction, to occur. That's why you never see browning in boiled food.
Brinnb is correct, but the other big difference between boiling and frying is that vegetables already have a lot of water in them. Boiling in water doesn't really change that because everything is already surrounded in water, but frying in oil is hot enough to boil the water inside the food, so in addition to the Maillard reaction, deep frying tends to dehydrate food. This is one of the reasons the outsides of french fries get crispy, and taken to the extreme, this effect is used to dehydrate ramen noodles in the production of instant noodles.