My Items Were Stolen From My Check-in Luggage After SunExpress Airlines Flight

I had a flight with SunExpress Airlines, a joint venture of Lufthansa and Turkish Airlines. I handed over my check-in luggage fully closed, but when I received it back, it was open, and my valuable belongings were stolen. Items missing from my bag include a JBL Clip 5, an Apple MagSafe 3 cable, a 96W Apple adapter, and two pairs of sunglasses. SunExpress ground services refused to let me fill out a form or provide a damage report. The call center directed me to submit a form stating that I had not visited the missing items office, which was completely untrue, but they forced me to select that option. Yesterday, I received a response saying they are not responsible. So if SunExpress takes everything from my bag and returns it empty, is that really my responsibility? Their service is untrustworthy, and their reporting system is terrible. If you are flying with them, avoid checking in luggage because you can get stolen from!