I will tell you why I love sand-boxy games.
It's because they put focus on player community, and social aspects of the game, rather then on theme-park elements.
Theme-park games isolate players from each other more, because an actual content is fully defined in the game files, and player community is just there to consume it.
In sand-boxy games, player community creates the good part of the content, and players work together much more then in theme park games. Community grows better, and game usually lives longer (not only being dependent on new expansions).
Now this is not only valid in PvE. It is valid in PvP as well.
Part of people might not agree with me here, but, in my opinion, in sandbox game PvP (same like in PvE), players are supposed to work together and act as community.
If few thieves are bothering someone, don't say "it's not my business", but rather help your neighbor, or when you will be the bothered one, your neighbor will ignore you as well.
People complain about PvP (thieves, murderers) a lot, while refusing to be a community that helps each other, just wishing to solo grind wood and fish.
That way, of course you will not have a good time. And it's your own fault. It's not the thieves fault.
Build your community, help each other, and observe how thieving in game doesn't floorish anymore.
If you are looking for a game where you will be protected from thieves by forced game mechanics, then honestly, sand-box games aren't for you. You might consider playing a theme park game better.
If you wish to play a sandbox game, then you will have to work with your community to protect yourself. It's just how it's supposed to work.
Playing a sand-box game, but complaining to devs to protect you from PvP... I'm sorry but in this scenario you are the problem, and not how the game works.
Thanks for reading.
(special note: all this is one players opinion, you are free to disagree)