M.J. Saulnier
Semi-Retired User
You pretty much hit the nail smack-dab on the head when you said that play-by-post roleplaying is more like interactive storytelling. When I think of roleplaying, three different "types" of roleplay come to mind.
1. The table-top roleplaying that you mentioned, which involves more acting.
2. The purely story based roleplaying that you see in most play-by-post roleplays.
3. And lastly, roleplaying that is somewhat story based, but involves a system. Sort of like a text-based game.
People get #2 and #3 mixed up pretty frequently, but its not uncommon for play-by-post roleplays to involve a system as well as storytelling. Actually, people do it all the time.
As for why people don't get involved with table-top roleplaying more often, I dunno. Its hard to say, everyone has their own reasons. But the main reason is probably because (most) people who enjoy play-by-post roleplaying are not very socially interactive people. At least, not when it comes to gathering together with a bunch of people to roleplay together. That's how I am personally, but I cant speak for everyone of course. Another reason is most likely because play-by-post roleplayers seem to enjoy writing more than acting. This is also true in my case.
I don't find there is any "acting" invovled in dice-based RP on forums. Tabletop perhaps.
I dind more depth and complexity is required of play-by-post RP.