Piece work has existed for at least 500 years[1] and those were definitely "flexible jobs that you could engage with at any point in time." Unsurprisingly, piece-work employment also suffered from many labor issues[2]. Uber is just piece work for giving rides, and so should be governed by laws that protect piece-workers[3].

[1] https://en.wikipedia.org/wiki/Piece_work#Guild_system

[2] https://en.wikipedia.org/wiki/Piece_work#Abuse

[3] https://en.wikipedia.org/wiki/Piece_work#Minimum_wage