Because the gains from increased productivity have been reaped by executives, not workers. You keep working the same hours to make your bosses more money, when you could be working less instead. This income gap has been widening since the 1970s[1,2].
[1]: https://www.cbpp.org/research/poverty-and-inequality/a-guide...
[2]: https://www.epi.org/publication/ceo-compensation-2018/