I think many if not most traditional RPG tables are the same group of people meeting to play the same campaign on a schedule. Your description of running 5+ simultaneous parties with various players moving in and out isn't (I don't believe) a good baseline to expect others follow.
The players are (usually) the same; just the characters are different.
For example, this adventure might feature a party consisting of characters A,B,C,D,E and F. After this adventure that lot gets put on hold and we jump to another party with characters G,H,I,J,K,L and M. After that those parties might meet and interweave (let's assume for these purposes all the characters survived), with the next adventure featuring characters A,H,I,C,D,L and M and maybe following up on things done/discovered in one of both of the first two adventures. Meanwhile B,E,F,G,K and L either stay in town or find themselves something else to do, which would get played through later.
Player 1 runs characters A and G.
Player 2 runs characters B, H and I.
Player 3 runs characters C, D and J.
Player 4 runs characters E and K
Player 5 runs characters F and L.
Character M is an adventuring NPC.
Lather rinse repeat for a decade or more, with players occasionally joining or dropping based on real-life situations and characters coming and going all over the place; and now you've got a deep, wide campaign.