Wikipedia:

Retard has transitioned from an impartial term to one that is negatively loaded. For this reason, the term is now widely considered as degrading even when used in its original context.[10]

Much like today's socially acceptable terms idiot and moron, which are also defined as some sort of mental disability, when the term retard is being used in its pejorative form, it is usually not being directed at people with mental disabilities. Instead, people use the term when teasing their friends or as a general insult.[11]

Do you think people who have the condition of mental retardation experience bigotry in our society? Are you not aware "retarded" has been used extensively as a slur? And are you so inconvenienced you can't come up with a single less-problematic synonym?