That is because actions you perform are too simple in most games. For example, both Warband and Path of Exile are games where you control trained fighter and gameplay of both games consists of combat and killing the enemies.
Problem is that in Warband actions needed to kill an enemy give you so much freedom compared to PoE. After 1000 killed enemies in PoE I'm bored beyond belief. In Warband I can endure 10 times that number and even then I'm not bored, just tired. Because of that I can play Warband whole day, while I can't play PoE longer than two hours in a row.
Same goes for sandbox game, if crafting is implemented in a way where you combine some items by placing them on some grid like in Minecraft, of course that will bore you quickly. Especially if you need to wait half minute in order to obtain resources (farming/mining part) like in most MMORPGs.
That's just it, the core gameplay and game pace trumps all. I was in the same situation with PoE - to kill things you need to press 2 buttons and a mouse click, with no special mouse moves, extra aim or anything like that. It got too boring for me to continue after only 20 levels.
While in games like cRPG or Quake you don't press many more buttons, it's the core gameplay and their fast pace that make them fun. You have a lot more stuff to control and watch over, your attention/reflexes are at hard work and your mouse/aim skills are at a great test - this coupled with fast paced gameplay is what makes it fun. Popularity of Quake Live is a great example of this, because there is nothing to "grind towards to" there - people play it because the gameplay itself is fun. As for cRPG, I doubt so many people would grind so much if Warband didn't provide an interesting and engaging combat system.
Sadly, there are absolutely ZERO full sandbox games that have core gameplay/combat system that feels like it's worth staying for, thus they eventually all feel like grind.
Still better than the bucket grind on h&h 
I think that a team of active players can archive quite a lot, and fast. I remember the Fallen settlement we had on h&h long ago. In a few weeks we grew extremely strong. Too bad only Vibe was actually good at that game + the raid that made everyone rage quit happened really fast. Some 300strenght guy mauled himself in through our palisade, and then everything got destroyed.
Two Russians actually lol. We were quite newbie at that point, no brick wall that would've saved our butts in this case (which is pretty easy to set up for a group of
established players, tbh). You can read more about it here
http://www.havenandhearth.com/forum/viewtopic.php?f=25&t=19902 
But what I really wanted to say is that having now played both Salem and H&H extensively, I can without a shred of doubt say I enjoyed H&H more. Yes it was still grindy, but the character progression was more fun and less of a chore (the system after bucket grind ofc). Salem's is way way WAY too grindy - spamming countless of inspirationals. The curiosity system of H&H is IMO just worlds beyond - you can't/don't have to spam it and it's time limited.
I'm going back to H&H as soon as the lag is fixed. I can't stand Salems ugly ass 3D anymore - imo this was a bad design choice by devs/Paradox, because 3D doesn't automatically equal good. The 2D isometric of H&H was way more pleasing to the eyes.
Salem has a few good stuff that H&H could learn from (no waking up at X hour to refuel steel torture), but overall I'd say H&H is simply better.
