Sure, but nothing ties eBay sellers to eBay. For an Uber driver to add another Taxi app it's not just a case of downloading and installing the app. They have to create a user account, enter their bank account details, verify their email address, take a photo of themselves, their car, drivers licence and insurance documents and upload them to the Taxi app server. Then wait a day or two for it all to be processed.

If the Taxi app has a significant number of users you might convince driver's to jump through those hoops. But if the Taxi app has no users it's impossible.

All the taxi apps are incentivizing signups by subsidizing their pay rates. Everyone is essentially taking losses to grow market share. Drivers will signup for whoever can make them money, so there isn't as much friction as suggested. They earn their livelihoods from driving, so a couple day sign up process is well worth it.

Uber was flat out paying drivers to sign-up.

This is an easy problem to solve.

the eBay analogy would be that nothing stops people from posting the exact same product on multiple sites at the same time. If I were looking at something on eBay, and I could do a quick google to find that same product on another site for 15% less, eBay will very quickly find themselves in trouble.

In India at least, Uber is asking drivers to do ~5hrs shifts at a time. Or they cut the amount given to drivers. This is due to fierce competition from Ola, Meru and other cab companies.

Not sure, when they will just have to accept that those drivers are their employees(maybe a lawsuit is needed in India as well).