Bloomberg Software Engineer interview questions

Applied online, got resposed about two week later. There is a phone screen before the onsite. The phone interviewer is an indian guy. For this round, half of time focused on my background and my research project. The remaining time, he asked me a algorithm question, which is matrix problem similar to the one listed in the leetcode. I did not gave the optimal solution. Then he asked me some oop concept and let me give him a toy example about the virtual function and inheritance. I was told that i passed the phone screen. An onsite interview was scheduled one month later. I made it to the second round but got sent out after that. The interview in each round follows the same pattern. First, the interviewer asked me talked about a project listed in my resume, what's the challenging part, how to improve it. Then a technical question was asked. Then I have to write the code on the paper, which make me very unconfortable. The question is trivial. One is check if a string of parensis is valid or not. The other one is check if a BST is valid. Overall, the interview process is not bad, but not confortable eigher. Util now I still don't understand why then rejected me.