OP, you are so wrong, these aren't just platforms were you earn money by performing tasks. These are freelancing platforms where companies and other employers outsource  their projects to qualified professionals,  and they do this from the economic point of view, possibly to cut down the cost of employing and maintaining an in-house professional. You don't just do tasks, you bid for these contracts, the employer verifies your capability and outsources the contract to the most qualified professional under some agreed upon terms which includes project cost and payment schedules based on milestones or at the end of the project, depending on the agreement.
Its in no way tasks, you give tasks to someone already working for you, but contract is for someone whom you want to work for you for a specified length of time.