I don’t think American girls are any more reluctant to marry an Indian man to help him get a green card than they are reluctant to marry a man from another country. It is illegal to marry just to help someone get a green card. It is considered immigration fraud and you can get into a lot of trouble if you are found guilty. Also the process you are required to go through is not simple. First, you have to actually get married. Then you have to file for a visa and include a copy of your marriage certificate. After several months, you must both be interviewed where you will be expected to show evidence to demonstrate your marriage is real.
You will be asked to answer a lot of personal questions about each other and to show documents that prove you have joint bank accounts, car ownership, rental or home purchase receipts, photographs of the two of you together at family events, and so on. After two years, you have to file another application and go through a second interview. It is not an easy process and the girl is likely to get caught and the man is likely to get deported.