Maybe I don't see the relevance of your question then.
Software developers are not negotiating fixed price contracts with their project managers every two weeks. Just how toxic such a situation would be should be obvious. Managers need to understand the context of the job and determine if they're satisfied with the productivity of their employees.
Agile is a development process not a contract pricing strategy. If anything proponents of "Agile" would be more hesitant to quote you a firm fixed price contract than "Waterfall" teams.
They don’t have to hire you every two weeks but they are paying for you and do have to decide how to allocate your time and plan various projects with interlocking dependencies. How can they do that without even a rough idea of how many people it takes for how long to accomplish anything? You are, in effect, still asking them to write you a blank check to accomplish something when they might want to change gears if apprised of how difficult it is.
If you go way back up in this thread the reason given for Scrum being bad is it features estimation, which is useless because it's inaccurate, so telling me that systems other than Scrum feature estimation is not a rebuttal.
Prices in the real world are barely linked to costs. Budget risks are bundled in markets with high variance.