You make the mistake of assuming that the outcome of the rewards is evenly distributed. 

Normally the better rewards have a lower chance of being awarded than the useless ones...

So you are not running out of luck...with some of my secondary accounts, I had to run hundreds of 7.x to get even one of those artifacts...

They will eventually drop...and it looks like you got a good approch to it by leveling food at the same time.

According to HH's stages tool, the odds of getting a 4* artifact on hard difficulty is 10%: 8.5% common, 1.5% uncommon

So a 1-in-10 chance, before you split it 8 further ways for the primary stats

Thx for the answers. Clear it is (to) hard to obtain the 4* artifact with a certain primary stat. 

May I ask the devs to reconsider the difficulty for this kind of objects, because I am sure a lot of players will pass to get it and in this way it blocks further progress.

The thing is, you've getting XP and silver all the time you do these quests. They can sometimes get annoying, but there's no point feeling that you need to rush into them because there are always progression bottlenecks. 

Honestly I wouldn't worry too much about the challenges. Put some of your daily turns into it and do the other stuff. 

After x-times trying, finally got my first 7.x challenge, so there is hope for the world 😁

Congrats!