I have a question about a verse in the quran?

in the quran in the surah al baqarah it says this "Allah has set a seal on their hearts and on their hearings, (i.e. they are closed from accepting Allah's Guidance), and on their eyes there is a covering. Theirs will be a great torment." my question is why would god want to set a seal on their heart and not heal them if he is the most merciful. thank you and only nice answers please coming from a curious muslim :)