I guess the short answer is that they're crazy. And I guess I do see a lot of crazy looking homeless types wearing thick fur-lined jackets on sweltering days. Fascinating how being crazy could make you so immune to your own body temperature though. 
 
But it's not just the street crazies. Sometimes it'll be a godawfully hot and humid day here in Manhattan, in the region of 90F with 65-70% humidity, and I'll see some girl walk by wearing a thermal fleece done all the way up to the neck. What's up with that? I remember one time going down into the subway and it must have been a crazy humid 115 degrees down there, and there's this Scandinavian looking tourist on the platform wearing a thick oilskin coat fastened to the top. There had been a little light rain about an hour ago, but it had since cleared up and yet there she was, in the kind of humid heat that makes the sweat roll down your back constantly and makes you want to tear all of your clothes off, in the kind of jacket you'd wear in a winter storm on a North Sea fishing boat. The fact that she hadn't even unfastened it a little was what really got me. Some people are just plain freaks. Or is it a circulation problem or something? 

I've wondered that myself (I can get too warm in 70 degrees F).
 
Maybe they're Zombies? 

My sandals come out of the closet when it gets past 85.
 
And no I don't mean like this:
 

New York City summer screwed my sense of fashion and my wardrobe.
Las Vegas finished what was left of it.
 
Haven't worn a tie in over 2 years. :(
 
But my first summer in NY, I could have been one of those folks. :P
 
 
 

By the end of the summer here I am so ready to start wearing sweaters and jackets and shoes again. And to be able to wear a single set of clothes during the course of the day without having to change.

Homeless people do it because they don't have any place to store it for the summer and if they just leave it somewhere, another homeless person will take it, then they won't have it to wear when it gets cold. 
 
homeless tend to carry everything with them that they care about.

Also Hipsters....
 

one of my sons wears a coat all year round.  usually a black leather trenchcoat.  because he thinks it's cool to do.
 
I can tell you, tho, he STINKS all the time!  he reaks of BO and I keep telling him it's because he's wearing that stupid coat all the time, but he won't listen. 

Beagle
one of my sons wears a coat all year round.  usually a black leather trenchcoat.  because he thinks it's cool to do.
 
I can tell you, tho, he STINKS all the time!  he reaks of BO and I keep telling him it's because he's wearing that stupid coat all the time, but he won't listen. 

[/quote
maybe he is emulating medieval times?
That's why weddings were held in June then, since you only took a bath once a year .
 
I guess you didnt want to offend your mate with BO  during the consumation

Remember Billie Crystal's great tagline from SNL - "it's much better to look good than to feel good"?
 
I've even seen this irrational fashion in the Philippines, where hip kids wear stocking caps pulled down over their ears as if they were walking to school in Minnesota in January. I shudder to think what that headwear smells like at the end of the day.