Why do parents reward their children only when they are good?

I know I am quite childish....teen rant ahead. Please ignore it you don't like this type of stuff
So basically in my country I am in 12th grade right now and I have my entrance exams next year. In 10th I was kinda like a topper and scored high marks. Many of my friends used to talk about how there parents would scold them for their marks and stuff and I never really understood them, I always told them how our parents love us and want our future to be good and my parents had always told me that marks don't define a person and I could be whatever I want. They gave me a huge cake after my 10th final exams. I love science so I chose that stream and 11th is difficult so my grades dropped a bit (borderline B)...my parents asked me everyday if I didn't feel like studying anymore and that I should study more. I could have told them that 11th was a bit tougher and I was getting acquainted with it but the image I had of them just shattered...I felt like they didn't love me anymore (ik they still do but...) and other stuff had also happened before (maybe some other day). I didn't get any cake that year. I really love science and I feel like I will come to hate it if this continues. With all this corona stuff schools have closed and I feel really bad at home. I don't know what to do.
I just wanted to rant somewhere so...sorry if this was too long
Do share your experiences and if there are any parents on this site please correct me if you think that I am wrong.