Please explain why they needed to rob banks...

They robbed branches of the same bank to raise enough money in order to by back their family property from said bank. It was their way of avenging the injustice of a financial institution that wronged them.

I don't recall any character in the film who was an oil investor, so evidently that option did not exist in this story. Just as well, it would go against the brothers' interests to reveal the true value of the land before it was 100% officially their property.

My impression was he only found out about the reverse mortgage after his mother died which set a ticking clock to the end of the month (the "she died 3 weeks ago" line to the ex-wife). His attorney clearly stated the money was given to the mother herself so her son wouldn't have had knowledge of it until she died. To get an investor, it would most likely take more than this time limit too - it doesn't happen that fast. You have to provide all kinds of proof of what you are saying is true - otherwise a person could end up investing with a JMACK scam style investment.

So either way there is a ticking clock and he had to make a decision and work fast. Robbing the banks also spoke to a self-reliance other than expecting a "bailout" a concept repeated numerous times in graffiti and other signs throughout the movie.