There are different player characters out there, some of whom may quickly resign a game of Go, while others will continue playing even after suffering a severe loss. The later is not without reason: Even if some of your stones die, it does not mean they are worthless.
The point is, that many dead stones leave tons of good aji to be exploited. Once you realize that, you realize that you still have decent chances of winning games that you would otherwise have wanted to give up.
Also, estimating final score is actually quite hard in the early stages of a game of Go. I cannot count the number of times I've been surprised by the final score of a game. Like, I was thinking I would loose, but then a win would be counted, and vice versa. I just had such a game with an opponent who resigned after less than a hundred moves, because he lost a big fight. Nevertheless, when I counted the game by playing it out, it was still him who should have won the game.
Thus, if you make a habit of resigning games early, you are not playing to your full strength, you are loosing games that you would have won.
Many Go players know this, either explicitly or intuitively, and are slow to resign consequently. The ones who don't know this unfortunately have a hard time realizing their mistake, as they never see how their resigned games would have played out.