Kindly note these points as far as I understood the puzzle:
– Only Steve is choosing the seat randomly.
– Everyone else who is entering the plane is NOT choosing the seat randomly. As first preference, they are occupying their own seat.
– Only scenario in which a passenger is not likely to get his/her seat is if Steve has occupied that seat

Now let’s get probability calculation
Probability (out of 100 seats), that Steve occupies any particular seat number is 1/100 = 1%
Probability that Steve sits on his own seat by chance = 1% – But this probability is NOT important in this puzzle
Probability that Steve occupies my seat by chance = 1% – This is the only scenario in which I lose my seat, in every other scenario, I get to sit on my own seat
Probability that Steve does not occupy my seat = 99%

Thus probability that I get to sit on my own seat = 99%