Here's why:
* Historically: They are euphemisms for stronger words like "hell" and "damn."
* Cultural Context: Their offensiveness can vary depending on social context and individual sensitivities.
* Frequency of Use: They are used relatively frequently, which can desensitize people to their "badness."
In conclusion: While they are technically swear words, "heck" and "darn" are generally considered acceptable in most social settings. However, it's always a good idea to be mindful of your audience and the context when using these words.