Why do Muslims abuse non-Muslims by calling them "Kafirs"?

‘Kafir’ means one who rejects. ‘Kafir’ is derived from the word ‘kufr’, which means to conceal or to reject. In Islamic terminology, ‘Kafir’ means one who conceals or rejects the truth of Islam and a person who rejects Islam is in English called a ‘non-Muslim’.

If we can use 'Non-Muslim', then we can use 'Kafir' in arabic also. 'Kafir' is just the Arabic meaning of 'Non-Muslim'.

If any non-Muslim considers the word ‘Kafir’ i.e. ‘non-Muslim’ as an abuse, he may choose to accept Islam and then we will stop referring to him as a kafir i.e. a non-Muslim.