Except taxis also experience network effect. There is no central 'hail a taxi' telephone number(at least in my part of the world, might be different in NY), every company has its own numbers, own dispatch, and own rates. They all compete with each other, and while some bigger companies might ride on the too lazy dont care just come and pick me up tail of the market, majority still chooses their ride based on the rate and previous experience (nice driver, clean car, speed).

In Europe small scale transportation is as competitive as cellphone market. Hmm now I am beginning to see the problem, in US you might not understand what a competitive cellphone carrier market looks like in the first place? :) Here you can transfer numbers freely between carriers, there are usually 3-5 providers to pick from and they all compete fiercely on price, level and quality of service. Changing companies is not a mayor ordeal, merely an ~hour of work to optimize your phone bill.