Lots of reasons. Just beware on marketplaces (Flippa, EmpireFlippers) etc. which have varying levels of vetting (from none to basic verification) as sellers often make up reasons or are lying about hours worked.

Some legitimate reasons to sell something "passive"

- Have another business growing that needs cash - Need a lump sum of cash for X - Lower tax rate (generally) paying capital gains vs. income tax - Cash in the bank often beats future uncertainty - Being passive doesn't mean it's low risk or doesn't keep you up at night