They do. But their income includes revenue from taxation. Suppose you own a London business. You pay taxes to the local government. In London, these taxes include money for TfL. After all, you chose to put the business in London, a city with a large public transport network, if you didn't like that you could have put your business in, say, Slough, and avoided this cost.

Passenger revenue is TfL's largest source of income by some distance, but it's nowhere close to enough to pay for the entire network, let alone the necessary capital investments to grow and change with the city. And of course if prices went up, ridership would fall, and transport would be diverted to the over-stretched and polluting private transport options which the city does not want.