You're underestimating their lack of bargaining power and the power differential with the employers in the region.

At least a feudal lord had obligations to their subjects.

you say that as if the cost of getting a different job was zero, where in fact if there's only one employer around and you have a family or something similar, it's more likely to be in tens of thousands of dollars if you include future medical bills for stress-related illnesses. it's very expensive to be poor and/or out of options.

My understanding is that one of the primary differentiators between serfs and lords was that lords were able to own land.

Since many (most?) people in the US cannot afford the land and other resources necessary to provide for themselves (e.g. via farming or craftsmanship), I'm not sure that it's unfair to draw a parallel between them and serfs.