If someone needs that money to eat they'll do the job, but if you're asking them to wreck their body in inhumane conditions in order to have slightly more spending money then they're going to say no. Even if their living conditions are lousy it's better than bending over in the mud under the boiling sun while a slave driver yells at them all day long.
Perhaps you could try paying them more?
Seriously, I can do this all day.
To address your question as why this is allowed to continue, you'd have to ask those within the US and countries that allow it to happen.
Australia addressed this in 1907 or so with the Harvester decision and other related law cases that became foundation to working conditions.
https://www.fwc.gov.au/about-us/history/waltzing-matilda-and...