Taxis drive a lot of miles and they drive way more miles per day and year then an average normal driver would put on their cars.
Taxis drive an average of 70,000 miles and sometimes more per year depending on where the Taxi is operating.
Compare that to the average miles that a regular driver puts on their cars which is around 12,000 miles per year.
Taxis are usually well maintained too and they drive them until the wheels nearly fall off meaning they get all the use they can out of each Taxi before it's sent off to the scrap yard.