Why not? The entire shtick of Uber et al is that employees choose their hours and gain "freedom" from employees. This is what freedom looks like. Employers can't tell freelance contractors how they should behave. They decide a contract, they get a service and get their job done.
The contract can absolutely require exclusivity if paying by time.
See: https://www.irs.gov/taxtopics/tc762
It's mostly not whether they can contractually demand any one particular thing. It's the sum total of behavioral and financial control they exert. Exclusivity would be a pretty big ticked box.
If Uber/Doordash tick enough of those questions, then they are at risk of having to treat their drivers as employees. Which is more expensive.
You're saying that they should get both the benefits of freelancers (pick work) and employees (minimum wage).
Other than the obvious authoritarianism, that would also harm the environment with people driving a lot more.