Or, you could just as glibly say: those who could, did, and those who couldn't, copied.
I have no idea if that's actually how it went down, just as I presume you have no particular evidence this is a nuisance suit. But if Costco did indeed copy Acushnet's patented features, I take it you wouldn't deny the actual inventor legal recourse.
IIRC, Costco bought up a contract manufacturers overrun (company was hired to make X number of golf balls, but for what ever reason they made Y number).
So the Titleist folks hired a manufacture in China to produce 3 million golf balls. The Chinese company either made 6 million or the contracting company rejected the lot as inferior. Either case the Chinese company now has 3 million golf balls that it doesn't want to lose money on. So they sold the whole lot to Costco. Costco then goes and sells them $15 / dozen
.. versus Acushnet's $45-$60 / dozen.
Acushnet sees its gravy train approaching a washed out bridge and files lawsuit to repair it.
If this was true, then there would be a case because the balls are slightly inferior but otherwise identical.
But Costco is arguing they're different. And knowing that the Kirkland store brand is actually quite a good one, I'd be surprised if Costco went with 3rd shift manufactured balls. Costco is not Walmart, and in general their store branded stuff is of great quality and manufactured properly, not low end cheap Chinese made stuff.
So Costco likely went with another high quality ball manufacturer (which may or may not be made at the same factory, but not 3rd shift production) and made those balls.
The reason Costco sells them cheap is because they deal in volume - instead of making balls in hundreds of thousands, they can make balls by the millions, extracting mass production cost benefits.
And because they were partnered up with another company who designed the balls, they got a good quality ball, made quite cheaply in volumes that out-do the other manufacturers since Costco does stuff in bulk.