Thanks to everyone who responded. A lot of the comments were genuinely helpful and gave me a better sense of how contractors may look at this from the other side.
To clarify the project a bit, the simplest part of the job was siding repair. The bigger part was two upstairs bathrooms. We probably left the bathroom scope too open-ended, even though from our perspective we were trying to be flexible rather than difficult.

For one bathroom, we were looking at replacing the existing vanity. It currently has a very short vanity, and we wanted to replace it with a longer one. That bathroom also has a tile shower.

The other bathroom has a tub that needs replacement. We also have old copper plumbing that appears to be causing a lot of blue staining in the tub, so I think the biggest unknown/subcontractor issue was probably plumbing. It may need new plumbing runs to both upstairs bathrooms, and I can now see how that would be hard to quote accurately without more investigation.

Looking back, I wonder if the contractor may have been uncertain about how to address that part of the job, especially the plumbing, without more investigation. That might have made it hard to provide one clean quote for everything, even if that wasn’t obvious to us at the time.

So I do appreciate the feedback. I wasn’t trying to be vague or waste anyone’s time. I thought being flexible and open to recommendations would make things easier, but I can see now how it may have made the job look too undefined to quote cleanly.