Its what I call 'Gripping On'

Why do authors do this with their series?

I  like Morganville series a lot, but take a look at this:
"BLACK DAWN is a staggering #12 in the series, but it’s not the last! In fact, there will be at least 15 novels in the Morganville series, and the next, BITTER BLOOD, is already under construction (and almost done)."

See that?
Once authors make a bang with their books, they don't want to leave their bloody series alone!
I orignally thought this #12 book, Black Dawn was the last in this fabulous series, but NO, it will its now escalating to a whopping 15 books!

I get it, almost most of the mysteries are solved... the machine that keeps Morganville safe, why the vamps chose a town in the sun as apposed to their 'health reasons', the bishop diseas scenario..oh lots got solved, and now what Rachel is bringing in the #13th book?

GHOSTS

Whatever!
Remember Vampire Diaries?
Exactly! That series never ended!
There was the Vampire Diaries four books, and then the Return sereis camo out if I'm not mistaken, and probably the books' still going on now with the Tv series version are also out in books...

Why can't authors just leave their books 'fresh'?
Even if it is ten books, make a blast into the market, and make a dramatic exist, rather than this stale slow never ending death of a promising sereies.
Instead, now they will eventually just wither into oblivion with the rest of 'incapable of ending books!'