Why do people walk around in shorts in Seattle?

Either you fit in the INTP personality type / INTP culture or you don't. 

It's a drier climate so you don't feel the cold like back east. Even in wet Seattle!

When my family and I lived in the Pacific Northwest near Seattle, my teenage daughters determined there was a Northwest uniform in all but the coldest, or even rarer warmer, weather. It involved shorts, a long sleeve jersey, and sandals with socks. The colors were almost always dark: greens, blacks, blues, and browns. Sounds as if much hasn't changed. 

"Cold" is a relative term with no specific temperature.  What you might find cold, others may find to be refreshing and pleasant.  Example: for me, an outdoor temperature of 17C is considered 'cold' to some but I find it very pleasant.  

I'd politely suggest that you not worry about other people wearing shorts and t-shirts.  It seems that you Yanks have about 8 gazillion other problems to worry about.

What's cold to you may not feel so cold to them. They're used to it.

Just because YOU are cold does not mean that everyone is. When I was in New Orleans, I was walking around in shirtsleeves when the natives were wearing overcoats. I got my comeuppance when I went from there to Michigan and nearly froze off my cohones.

As someone who has lived there were kind of both.