Fuel does not burn well in the cold cylinders. Some condenses on the cylinder walls and gets past the rings into the sump where it dilutes the oil. Water, acid and soot accompany it. Engine oil includes additives to neutralise combustion products, but cheaper oils are more marginal in their capabilities. Changing the oil regularly is the best way to help.
Taxis are well known for running huge mileages before engine rebuilds.
A taxi engine might spend its life running almost continually under near optimal conditions. Any fuel and water that does get into the sump is evaporated by the heat. This is caused by the accumulated fuel and water in the sump, that was masking a slight usage of oil, being evaporated off on the first long trip. Engines used in racing operate under quite abnormal conditions, running at full power for most of their short lives. They need an oil able to stand extreme temperatures and pressures for a couple of hours.