You are perfectly welcome to start your own mass transit system (rail, bus, boat, helicopter, etc). Chances are that you'll quickly find that between he high capital investment, highly variable fuel prices, environmental requirements, and demand challenges that it will be really hard to turn a profit.
Hence, not very many private entities want anything to do with the market.
Well most cities have subsidised public transit, so it is pretty hard to compete, even if it were legal (in cities like my own it is 100% illegal).
Variable fuel prices doesn't really change any of the other private mass transit systems, like air, ship, and train (or even taxi?).
Environmental requirements are essentially irrelevant, especially compared the the fuel that will be saved.
High capital investment? I could start with a single van and one frequently travelled road. Or provide the service exclusively to one employer (ala Google style).
Demand challenges meet multi linear optimization. Our fire stations, electrical grids, gasoline stations, highways, office buildings, and eateries all have demand challenges.
Why would the profit be especially lower? If it were too low, entities would leave the market, if it were too high entities would enter the market.
