The point is that employers have the power to define the hours worked and the wages received, and they will always maximise the hours work and minimise the wages paid.
Workers can only change income by applying for more jobs, or by having exceptional skills. (Which may or may not be genuinely high-value, but do have to be perceived as high-value.)
Since most workers don't have high value skills, that means most people have to work multiple jobs to survive. From the Marxist point of view, it's completely irrelevant if someone is working two shifts for two employers or one very long shift for one employer.
What matters is the loss of personal freedom and opportunity for the worker, who not only gives up personal time, but also lacks both the finance and the time - never mind the energy - to develop any independent small business prospects.
The political reality is that there by the end of the 19th century there were aggressive anti-capitalist movements working hard to limit hours and raise pay against very aggressive - sometimes criminal and murderous - pressures in the other direction.
These movements were less successful in the US than in most other countries, but they still had some influence - which had mostly faded by the end of the 20th century, and has now reverted back to the 19th century ultra-exploitative model for most of the working population.
The shocking thing about the US is the extent to which these movements have been written out of history. Instead of an accurate historical account of what happened, US economic and business history seems to have been rewritten entirely by propagandists who not only ignore the influence of worker movements on pay, social mobility, and working hours, but also use spurious pseudo-economic arguments to pretend that the oppressive policies that benefit the ownership caste also benefit the rest of the country.