Happened to us in Berlin on my visit. We were seated with two elderly ladies. Their English wasn't great but they were very friendly.

funny that you say that. i just noticed that twice today in a cafe. why frankfurt of all towns? munich makes sense for this to develop because of their biergarten culture with long benches and long tables

I used to live in Berlin and in my experience it wasn't really a thing there. For some reason it's just part of the restaurant culture in Frankfurt.

Not too uncommon in Bavaria and Austria, if the restaurant (or especially beergarden) is quite full - the waiters will even sometimes just place you there or give you the option to leave, or wait. I wouldn't say I've seen it if there are free tables though - but I've also been to regions where this would never ever happen, where your table is yours and yours alone.

I'd say talking to the people at your table is purely optional, sometimes it works, sometimes it doesn't.