how come?
any
bodyone can add you to their world and you can't get remove yourself, so your worlds list gets filled with junk
any
bodyone can copy your world, so they can easily go into creative and find where structures like dungeons, temples, villages and peoples homes are
if you want to add mods, or have a custom modded server, you have to fill out a form, then they go over the mods list, then they ask people if they would want to use those mods, if nobody but your group of friends wants to use those mods, you can't use them
you're better off hosting your own or buying a small server for mods