дважды надо прописать exit. не пойму где ошибка

Если ввести "exit" - программа завершается как и должна. Если ввести buy - back - exit то программа снова выдаёт приветственную фразу из функции main и для завершения надо ещё раз ввести exit. Подскажите, пожалуйста, почему не срабатывает первый exit после buy - back

def remaining():
    print("The coffee machine has:\n" +
          str(water_tank) + " of water\n" +
          str(milk_tank) + " of milk\n" +
          str(coffee_tank) + " of coffee beans\n" +
          str(cups_tank) + " of disposable cups\n" +
          str(money_tank) + " of money\n")


def buy():
    beverage = input("What do you want to buy? 1 - espresso, 2 - latte, 3 - cappuccino, back - to main menu:\n")
    if beverage == "1":
        print()
        return espresso()
    elif beverage == "2":
        print()
        return latte()
    elif beverage == "3":
        print()
        return cappuccino()
    elif beverage == "back":
        main()


def fill():
    global water_tank, milk_tank, coffee_tank, cups_tank
    water_add = int(input("Write how many ml of water you want to add:\n"))
    water_tank += water_add
    milk_add = int(input("Write how many ml of milk you want to add:\n"))
    milk_tank += milk_add
    coffee_add = int(input("Write how many grams of coffee beans you want to add:\n"))
    coffee_tank += coffee_add
    cups_add = int(input("Write how many disposable coffee cups you want to add:\n"))
    cups_tank += cups_add
    print()


def take():
    global money_tank
    print("I gave you $" + str(money_tank))
    print()
    money_tank = 0


def espresso():
    global water_tank, coffee_tank, cups_tank, money_tank
    if water_tank >= 250 and coffee_tank >= 16 and cups_tank >= 1:
        print("I have enough resources, making you a coffee!")
        water_tank -= 250
        coffee_tank -= 16
        cups_tank -= 1
        money_tank += 4
    else:
        print("Sorry, not enough water!")


def latte():
    global water_tank, milk_tank, coffee_tank, cups_tank, money_tank
    if water_tank >= 350 and milk_tank >= 75 and coffee_tank >= 20 and cups_tank >= 1:
        print("I have enough resources, making you a coffee!")
        water_tank -= 350
        milk_tank -= 75
        coffee_tank -= 20
        cups_tank -= 1
        money_tank += 7
    else:
        print("Sorry, not enough water!")


def cappuccino():
    global water_tank, milk_tank, coffee_tank, cups_tank, money_tank
    if water_tank >= 200 and milk_tank >= 100 and coffee_tank >= 12 and cups_tank >= 1:
        print("I have enough resources, making you a coffee!")
        water_tank -= 200
        milk_tank -= 100
        coffee_tank -= 12
        cups_tank -= 1
        money_tank += 6
    else:
        print("Sorry, not enough water!")


def main():
    while True:
        action = input("Write action (buy, fill, take, remaining, exit):\n")
        if action == "exit":
            break
        elif action == "fill":
            fill()
        elif action == "take":
            take()
        elif action == "remaining":
            remaining()
        elif action == "buy":
            buy()


water_tank = 400
milk_tank = 540
coffee_tank = 120
cups_tank = 9
money_tank = 550

main()