21 days turnaround time waiting for my ownership documents

Went to we buy cars in Germiston 3 weeks ago to buy a mercedes Benz SLK cash. Got there and was assisted by a guy named Simon. The sale went okay as I was buying cash. Then he told me that they will be doing the change of ownership for me and send my certificate to my nearest we buy cars. Which I told him it is their Southgate branch. The car disk has expired last year already. It's been 3 weeks and I still don't have my ownership documents and therefore I'm not able to get a new disk or roadworthy certificate. I reached out to this Simon person and he was so rude on WhatsApp telling me about the 21 days turnaround time. Not in a polite manner that he showed me when I was buying the vehicle. So please tell me, did I buy a car to park it waiting for 21 days for my cars ownership documents. What is the 21 days for. I mean I can't drive a car with a expired licence disc and no roadworthy certificate. What is 21 days for. This is really sick and inconsiderate to your customers