Why are HR and companies so salty if you reject them??

I recently received an offer from an org. After multiple rounds I was told I was selected and we discussed numbers. However I did not receive any offer for a month. I in the meantime checked about company culture and found out it's not good and when I did get offer after a month, I rejected it within the same day. I did ask them to send over the offer since I was still pondering. HR called me post that and started calling me unprofessional that I rejected offer post receiveing the letter and why I did not reject it earlier. Was I unprofessional? And honestly most of the companies reject candidates left right and center without a second thought post multiple interviews.