Why is my learning curve so flat?

It doesn't matter if you play 10, 100 or 1000 games if you always play the same way, you can't improve like that. 

Like somebody mentioned, find the type of game that suits you. I dont mean (just) the type where you play better, but the type of games that you enjoy more. I know I play better the more I enjoy a certain game/position, and I'm guessing its the same for most (casual) players. If you know, or when you find out what types of games that is make moves (and perhaps choose openings) that lead to those type of games.

Another thing I've noticed about players at your level (and myself when I was at your level), is that they often calculate hoping/expecting that their opponents make a certain move even though its not the best move, and often a blunder. Try to get rid of that (if you do it). It may sometimes work at your level, but it doesn't always, and it isn't really good chess.

I'm not being a pessimist, nor was it a sarcastic answer.

You had stated "50+ improvement after every 100 games is natural I think for any person with reasonal analytic ability and concentration levels. So according to my calculations, 1200 + 50 * (800/100) = 1600 rating after 800 games. Am I right?"

Gaining chess strength won't come from a certain number of games played. Sometimes improvement comes rapidly, sometimes it never happens. And then there are those who improve, level off for a while, then improve and level off for a while again.

Here's a case in point. When I played at Mechanic's Institute there were a couple of players there. Craig Mar was a Master who rarely played anymore. His father Henry was rated 1500-1600 and played in every tournament and at local weekend tournaments. So according to your calculations, Henry should have been a GM and his son a 'C' player. Reality is people reach a level in chess, otherwise we'd all be GMs.

 I think there are 100's of players who played more than 1000 live chess standard games on this site. seen someone who played more than 15000 live standard games(don't remember his name)

I noticed most of your games are at the 15 10 time control. That's much too fast for you to be able to think carefully about all of your moves and your opponent's replies, which means you aren't developing the sorts of analysis and thought-process skills that are developed through playing slow chess.

I suspect that if you played fewer games at a slow time control (like 45 45, which is a common time control for slow play on ICC), and then really analyzed your games carefully afterward, you'd find that much more beneficial in terms of improving your game.

And of course, tactics, tactics, tactics.

You don't progress much by just playing; you progress by practicing or studying the things which need improving.  Play less games, and watch a video every day, and/or do tactics puzzles every day.  Read http://www.amazon.com/Moonwalking-Einstein-Science-Remembering-Everything/dp/0143120530

You don't get better play playing (although playing is important).  You get better by studying.  How many times have you reviewed a lost game, identified your major failing, and vowed to never repeat that mistake again?  Perhaps is was a more general error as in you don't understand rook endgames.  How many times in those instances did you read a book about endgames or otherwise study rook endgames?

50 points for 100 games is hopelessly unrealistic.  We'd all be grandmasters by now if that's all it took.

u are absolutly right