But being shown a goat can change the probability. If Monty picks randomly (which is not the original problem but is interesting to consider) then there is no benefit to switching.
[1] https://en.wikipedia.org/wiki/Monty_Hall_problem#Variants
a) Staying wins (Monty opens to a goat) b) Swap wins (Monty opens to a goat) c) First pick lost, no chance to swap (Monty opens to the car)
each of equal probability.