The game has sold in the millions of units. Not everyone plays all at once and many have probably stopped for the time being. Take a look at the comparison with Subnautica.
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Concurrent players don't really tell the story. That huge spike for Subnautica is when they came out of Early Access. Tons of old players probably tried it out to see what the finished game was like. 7 Days to Die will probably get a similar spike when the announcement that it is finished makes its way throughout all the players who ever puchased it.
Here is what Subnautica and 7 Days have in common that keeps the concurrent numbers lower than they might otherwise be: No focus on PvP.