Because people overestimate certain aspects of a job (salary cap and current roster) and underestimate other aspects like job security, how tough the division is, and a team's general culture.
The Saints have a strong FO culture regardless of Loomis' recent fumbles that coaches can trust won't screw them over. We also play in the weakest division in the NFC (maybe football?). If a head coach comes in knowing he's got at least fours years (something we are likely to offer), he can take his lumps the first season with the cap and roster and be set up in year two to do big things.
Anyone going to the Jets/Raiders/Jags could be getting fired two years from now, and prospects know it. Even the Bears is a questionable job. That division is brutal and Caleb Williams is a complete weirdo.
NE is head and shoulders above all the other open jobs.