Many other diseases and illnesses cannot be cured. For example, someone with diabetes might need to take insulin every day, but otherwise can lead a normal life. Someone who suffers from asthma may need to take an inhaler occasionally when their breathing becomes difficult. Not long ago, asthma was a dangerous and fatal disease, but these days, it is nothing more than an inconvenience. But if you are asthmatic, there is no "cure" as such.
It's the same with bipolar. Finding the right combination of drugs, and effective therapy, can sometimes take time, because every person is different. But once an effective treatment system is found, someone with bipolar can live a normal and productive life.