It's because they don't know you yet. It's easy for a gatekeeper like a recruiter to shut the door. The hiring manager, if they do actually like you beyond butt in seat, can get creative. Typically they just have a budget and a unit of work to accomplish, they get to fill in the details.
Also, trying to negotiate 25% less work for same price is not what I suggested. It has to be 1:1 relative for them to view it as equitable. They have a lot of things to think about when considering what's "equitable". If their plan was to grind you for 80 hours and pay you for a "full time 40" then they weren't being equitable or forthright either, and it's very common, so it goes both ways.
Also, trying to negotiate 25% less work for same price is not what I suggested. It has to be 1:1 relative for them to view it as equitable. They have a lot of things to think about when considering what's "equitable". If their plan was to grind you for 80 hours and pay you for a "full time 40" then they weren't being equitable or forthright either, and it's very common, so it goes both ways.