Why don't the Indian Courts punish the offenders very seriously?

This is the paradox of Indian Judiciary.

There is no straight answer to your question. But the definite answer is scattered in the law books. Firstly in Indian Constitution (Article 21) strictly says that "No person shall be deprived of his life or personal liberty except according to procedure established by law". By this, every court has to follow the procedure and cannot apply the common sense by passing the law.

secondly the procedure, based on Article 21 if there is a doubt before the court that the accused has not committed the crime, even though the crime was committed in public, court has to give the benefit of doubt to the accused and shall set him free.

Indian police and politicians knows these loop hole (I say it at loop of ocean not just a hole) in law . By the influence of politicians and police enough arragements are made for the defense lawyer to raise the benefit of doubt in favour of the accused.

More over, hostile witness by bribing, threat etc are usual practise.

Benefit of doubt is given to the accused, unless the offence is proved beyond doubt. Laws are good and adequate. Quantum of punishment is not lesser. The implementation is failing. The delay is the main cause. When once the trial commences, after a long gap, the witness would loose interest and mostly turn hostile. Prosecution agency is filing charge sheet after a long gap. There is no scientific investigation. The Lawmakers and judicial system know the problem and also the remedies. But not implementing. The general public also not cooperative as they fail to give evidence. There are several reasons, unfortunately no steps are taken to remove hurdles.