>>545644tbh that's only part of it: it's common even in countries like the UK where there's less of a service-animal culture, or where "no dogs except guide dogs" was a common sign in the past.
i think there's two other reasons, one more US specific than the other:
1. in America landlords are allowed a no-pets policy, but they have to allow service animals to comply with anti-discrimination law. having a pet probably does marginally help with your depression, and everyone's depressed, so why not get your therapist to sign the waiver saying fido gets to come with you to the only apartment in your price range? …but then once he's a certified service animal, why not take him anywhere you like? especially in cases like airplanes, where it's not unheard of for someone to fuck up and put the dog carrier in the unpressurized hold.
2. for regular businesses, rather than odd cases like airlines, you're looking at freefall in the number of people who go to the town center relative to even say 2005. nobody goes there shopping anymore when they can get the same shit cheaper on amazon. nobody goes to local government offices when they can do stuff by e-mail. side effect: fewer people go to secondary services like cafes because they're not already in the area for some other reason, they're at home and can eat there.
but you know who's always outside? dog walkers. you know a good trick to increase foot traffic in your business? ditch the "only service animals" requirement and just let dog walkers take their dog inside. they
do just happen to be in the area…