Two things. Firstly, patients are being harmed because hospitals are being forced to cancel planned operations and procedures, some of which people have been waiting years for.
Secondly, they just went on strike last year and won a huge pay rise, significantly ahead of inflation. The argument put forth by the BMA is based on bad data and misleading statistics.
There are significant structural problems within the NHS that need to be addressed, including some that directly affect the earning power of young doctors. None of which will be fixed if they keep going on strike.