Why don't they improve?

I have seen many members who played thousand of blitz games and  still have rating between 1300 to 1500. What's the reason? Should we conclude blitz doen't improve chess at all?

Not if they were 700-900 when they started. 

Besides, if you feel secure with a higher rating after only 100 games, you can consider that your provisional rating.

See a more accurate strength reflection after several thousand games.

Improving takes an effort that not all are willing to make.  Who can really argue with the logic behind "Chess is a game.  Why would I want to work at it?".

Too much blitz hardly does anything but reinforce bad habits. To improve, you should go through your losses and become acquainted with new ideas (to expand your horizons).

Obviously, some people get stuck at a certain level because of their talent. Some players will peak at 2600, others at 2200 and others at a different rating.

There are two viable explanations:

1.Lack of talent

2.Lack of study. 

i believe that talent is an outcome of hard-work and correct way of doing things. Blitz games actually ask for instant replies which moves away the players from thinking deep and analyzing variations. When people play blitz, most often they develop an intuition based on incorrect ideas and thought process. 

Blitz and Bullet generally don't help you improve your play. Me, and I think most players, play bullet games just for fun or relax ! Blitz games helped me a few times, when I had 30 seconds on my clock and I had to play difficult positions. In some of these cases, my oponnent didn't play fast and finally I won the games on time or he just blundered ! So, most players may not improve because they are playing for fun and they don't want to study, they just want to play !

Or, like me, they enjoy Blitz, but at my age I'm not going to get much better and I'm much better in longer games. eg my correspondence rating at USCF is about 1850, but my old brain has blank spots that make OTB and expecially Blitz much harder.

very helpful