Why does a man have the absolute right to divorce, while it is difficult for a woman to obtain it, forcing her to file a lawsuit and redeem herself?

The questioner's statement that the woman is the party most harmed by divorce, and that she must ransom herself, is not absolute.

A divorced woman is entitled to her full dowry if the husband has consummated the marriage, and her maintenance for the duration of the 'iddah (definition) (waiting period), and some scholars also entitle her to a consolatory gift (mut'ah).

If a woman requests a divorce due to harm from the husband, such as ill-treatment, and the husband refuses to divorce her, she has the right to refer her case to a judge to compel him to divorce her. In this situation, she retains all her rights, and it is not permissible for the husband to take any compensation.

However, if the woman requests a divorce without any harm from the husband, for example, if she dislikes him, the husband is not obligated to divorce her. He may refuse until she relinquishes some of her right, such as a portion of the dowry, with which she ransoms herself, in accordance with the Almighty's saying: {Divorce is twice. Then, either keep [her] in kindness or release [her] with good treatment. And it is not lawful for you to take anything of what you have given them, unless both fear that they will not be able to establish the limits of Allah. But if you fear that they will not establish the limits of Allah, then there is no blame upon either of them concerning that by which she ransoms herself. These are the limits of Allah, so do not transgress them. And whoever transgresses the limits of Allah - it is those who are the wrongdoers} [Al-Baqarah: 229].

A woman ransoming herself with her wealth in this situation is considered a just ruling. It is inconceivable that the husband would expend his wealth on the dowry and expenses and treat her well, and then the wife requests separation without any harm from him, without returning anything of what he has expended.