Why EXL, TCS , Wipro , Tata, Cognizant, WNS Not Accept Notice period

I am Suffering to Switch my Company has 1 year Experience in P&C Insurance, I have Tried a Lot and Get Some Calls from HRs But They always said Join in 7 days. When I said I have to Serve 30 days of Notice and Because my Father expired I am the Only Earning Member I can't able to take this Risk to switch without Notice , And Non of the Above HR Understand my Condition, Why HR Not Understand ?