Because we decided as a society that it is. That's why say, the concept of minimum wage exists, and plenty other labor laws. The remaining question, legally, is whether Uber is offering drivers a "job", as opposed to a software platform that they can use to be self-employed as drivers (which is Uber's argument).

If Uber is offering jobs to drivers, then certain standards must be met to avoid illegal employment practices. Then the next question is whether or not Uber actually violates those standards. The answer might be that it doesn't, life can be pretty thought for people in many professions that we have actually deemed 'fair enough' work. But if the answer is that it does, then Uber needs to change.

If 'the sharing economy' is not actually creating employer-employee relations, then it might still need to be regulated (perhaps under different regulations than labor laws), depending what society thinks of the mechanism as a whole. For example, if drivers are self-employed contractors rather than employees, then what gives Uber the rights to fix their wages? Is there such a thing as minimum wage for independent contractors? Who is responsible for things like health insurance for Uber drivers? We have plenty of answers for these questions for the kind of independent contractor that makes $50 an hour, not for the kind that makes $17 a ride. But again, it might well be that the deal as it is can be actually considered fair, compared with for example being a street market vendor or a temp worker at a convenience store. I certainly have met plenty of part-time/retired drivers in the South Bay that consider the deal a fair enough source of extra cash. The point is that we haven't figured out whether or not it is exploitative and that is the sort of question modern developed societies can afford to ask themselves and weight against the benefit of 'it creates some value for some people and pays the bills for some others'.

Edit: I'll add that I use ride-sharing services often, want them to continue to exist in some useful form and am firm believer of 'default allow' as it comes to new business models. But that doesn't mean nobody is allowed to look into whether some new business practices are exploitative or not or that there can be no harm in allowing any voluntary economic transaction.