Maybe it's because they've been working their shitty burger flipping jobs, saving up all the spare cash they have... and so want to know exactly how much the sessions are going to cost them, so they can do some basic calculations and see if they have enough. They don't want to get burned and owe money that they don't have.
That's the whole point. I'm offering a flat rate so they know coming in how much everything is going to cost. If I did hourly most band underestimate how much time they'll need to get shit done, and I'm not really getting paid for editing (because I don't like doing it with the band breathing down my neck.)
The other reason I've been trying to do flat rates instead of hourly is because the studio is my side gig. I'm in graduate school getting my MBA and I work for the university, so I can't be at the studio 7 days a week. I like to be flexible about scheduling. The problem is bands are stupid, they don't understand the recording process or basic math, and I spend more time explaining this shit than recording music.