For me, I have the option of opening in a new tab by using middle-click, or opening in the same tab with left-click, and I use both options. If stories opened in new tabs by default then you would remove from me one of the options I use, without providing any benefit.

Oddly enough, this has been asked about before, and there has been some previous discussion. Here's a search:

https://www.hnsearch.com/search#request/all&q=title%3A%28ope...

There have also been people who have supplied GreaseMonkey scripts and bookmarklets. You might like to have a look at a few of these to see what I mean:

https://news.ycombinator.com/item?id=3560941

https://news.ycombinator.com/item?id=2469065

https://news.ycombinator.com/item?id=4880204

https://news.ycombinator.com/item?id=5085659

https://news.ycombinator.com/item?id=1866268

https://news.ycombinator.com/item?id=3549314

https://news.ycombinator.com/item?id=5284186

https://news.ycombinator.com/item?id=2353291

https://news.ycombinator.com/item?id=3695029

https://news.ycombinator.com/item?id=516933

https://news.ycombinator.com/item?id=1621892

https://news.ycombinator.com/item?id=5919030