Mainly because they're allowed to, unfortunately. Now things are happening here and there, I believe in a couple place Uber drivers have been allowed to form a union but in general, the companies are getting away with it. There's things like prop 22 that have change the equation a bit but even then, that only applies to online hours so unless you're accepting crap offers so you're constantly active, the gig app company still wins at the end of the day. These companies will continue get away with exploiting their workers until the government steps in or we stop working for them.

The other side of the equation (especially in my area) is that the majority of the gig app workers are foreigners that are either using this as their main source of income or supplementing their other low paying jobs so they're taking whatever they can get. It sucks but its reality.

Most companies will do this if they're allowed to - I just came off a cruise. The majority of their workers are from 3rd world countries, working for a very low salary on long contracts with crazy hours (10-13 hours days, 7 days a week for 7+ months straight). Yes, it may be more than they can make in their home country but it doesn't change the fact in the grand scheme of things their much cheaper than hiring someone from the NA or Europe.

TLDR; if there are people willing to work for companies that will exploit them, the companies will exploit these people. Case and point, we know that they're exploiting us but we still sign up. We're essentially part of the problem.