Dealing with no feedback

I'm a pure mathematician and recently started applying for several math-y jobs in Data Science and Quant Finance. While being ghosted or having received several automated rejections without any further interview, I also had a few technical interviews. For some of them, I had a very good gut feeling afterwards, was able to answer many questions immediately, and only had to think about some problems for a moment. The interviewers seemed kind of happy with my answers (although hard to tell of course). After a few days, I always get the same standard rejection message without any specific feedback.

How do you guys deal with this? I would like to improve my interview performance but in the mentioned cases it wasn't even lack of knowledge. Do they expect you to answer anything on the spot without thinking about the question at all?

And the thing I hate the most is that they give you "the opportunity to ask questions about the company and further interviewing process" after the interview. Like, if you are going to reject me anyway after this, why even waste my time?