For example, in the Uber-or-Lyft 'battle', sure, there are and have always been private car companies and private drivers, but when you want a car you're likely to open one or maybe two apps on your phone and call the quickest/cheapest option. Similarly, Facebook competitors have a hard time getting off the ground because when you're the only one in your friend group on a new social network, there's no one to talk to.
Meanwhile, a new email provider has a small amount of work to do to catch up with all the security regulations, but then can just provide value to its customers without having to worry about 'defensibility'. A single app aggregating Uber, Lyft, taxi, and 3rd-party car services on one screen would open those up for competition. Similarly, a federated Facebook that provided various social networks in one client would let you pick your hosting provider, configure your own feed, and communicate with people on other platforms.