Why our politicians are corrupted and not bothered about welfare of the people who had elected them?

Answer 1

Because Power corrupts. That's a basic fact. It's a drug, if you got a taste for it, you want more, you want to use it, and you'll do anything to keep it...

Then there's the fact that those who have the money and other means to be "nice" to politicians are not representative of the people at large, and the political environment is so inbred that the negative consequences of being corrupt are far from dissuasive when compared to the advantages to be gained from accepting the bribes and playing ball.

The people are just votes, but once in place, the officials are not bound to them... in a bipartisan system, you'll always have a cushy place even if the other side wins, and since the other side is just as corrupt, odds are even that you'll keep the job, unless you spark a Watergate size scandal... There's just no incentive to actually care for the people, it's the party machine that you need to be well regarded by.