Tons of LE's come out all over the world on a weekly basis (when everything is open). Hardcore collectors trade worldwide.

If I didn't have friends - who I met through trading - covering me for releases in Asia, Europe and yes, Florida, my collection would be a lot smaller - as would theirs as I get pins locally for them. People who get seriously into pin collection - unless they are VERY rich, and there are a few of those - need to trade to expand their collections.
In the parks, most people start trading by buying a starter set/booster set. This contains a number of pins - usually 6 or 8. Very few people buy starter sets to keep them. They're meant to be traded. Often they'll feature one popular character - like Stitch or Tinker Bell - that a child might want to keep, but then they would trade the Goofy, the Pluto, the Minnie, etc. Pin trading at Disney (or so the story goes) started because an exec saw people trading pins at the Olympics and saw that as an opportunity for Disney. #genius
The other way people often start trading and collecting is by buying mystery packs - just like those packs of baseball cards you mentioned. So they wind up with duplicates and trade to try to finish the set.
And yet another way is by someone seeing a pin on a lanyard or on a board or in a book and saying "hey, I like that pin, how can I buy it?" and the person/CM replying "you can't buy this one, but if you get (buy) another pin, you can trade for this one that you like". That first trade is the 'gateway drug' to get you hooked. lol