Transit is different, you are going from one place to another. Cafes are where people that know each other come to socialize. You will almost never see a lone person in a cafe in the country where I live in, and a lively conversation tends to pull other people in as well.

It's just different so their initial thought of it being awkward without further reflection is based on reality not some blown up fear.

I also say this as someone that has no trouble striking up conversations with strangers. So not like I barely go out.