Who's absorbing the cost?

This is probably an incomplete answer since I was never involved in the distribution side, but in the old days, the major labels ate the cost of defects and returns and probably pushed back against the pressing plants if there were defective pressings; the inventory was distributed to major chains directly by the big labels and through jobbers and one stops to smaller retailers. My memory is, Casablanca Records crashed and burned, not only because of extravagant spending but because when "disco" music  suddenly died, the label had to eat all the returns. The old style recording artist contracts always had provisions for deductions from royalties for returns, not just defects. The artist didn't get paid if the record company suffered returns.
I suspect with the change in the record industry business model- and particularly vinyl- which is generally viewed as a boutique sideline at best- the inventory is sold on the basis of no returns--often these pressings are more limited in number, they aren't going back for re-pressings, and unless there is some major defect that affects a broad number of copies, the label isn't going to want to hear about it. The retailer will probably eat it as a matter of customer goodwill. 

I don't understand how the (lack of) quality control exists in multiple plants like this will all the defective pressings.

I remember the Old Days. Defective record ; you got a new copy of the same record. Do not ever remember getting refunds. I am a long time customer of Music Direct ( lived 20min. walk- 5min drive ) now they moved, 1/2 hr drive. Never a problem w/anything.

From the retailers named in the comments, I gather that most of the commentators in this thread buy their vinyl at a distance, using the internet or an 800 number.  When a record store is available, it does have one advantage in dodging defective records (or discs):  you can unseal and examine the album in the store in front of the salesperson.  The surface should be pristine.  If there's any question about a defect, a player should be available to test it.

I occasionally bought used CDs at Amoeba in LA.  Because the inventory was used, it was mostly unsealed, and there was no problem with examining the disc before you purchased.  Sometimes Amoeba would reseal albums that it considered to be in very good physical condition.  I wasn't persuaded, so whenever I considered a resealed album, I asked the salesperson to open the album so I could examine it.  I distinctly remember rejecting potential purchases on two occasions.

This is why I support my local record store. I'm rather lucky that here in Columbus, OH we have 5 or so locally owned shops. I'm in them enough that if there is an issue with a record they replace them. If they don't have any more copies they will usually offer me a refund, probably knowing I'm going to purchase something else anyway.