> The person doing orders of magnitude more work is probably only getting paid 10% more.
You've answered your own question. Transparency is undesirable from an employer's perspective because it encourages a closer alignment between compensation and productivity, and the mismatch between these currently serves as a significant profit advantage to the employer.
If I can get 1000% more work from someone, and pay them 10% or 100% more, I am highly incentivized to prevent them from becoming aware of the value of their efforts, because they might realize their (relative) exploitation.
At the same time, the less-productive worker also benefits from this lack of transparency because they get ~90% of the pay for 10% of the work. Disrupting the status quo would not be economically rational for them either.
Thus, only the small fraction of underpaid, overworked people benefit from increased transparency, and it is not surprising to find that the majority would resist its implementation.