Why do physical wirhdrawals from nicotene only last a few days?

Firstly, they don't: just last a few days, and can last weeks and even months. The worst of them last 3-5 days, which is true of virtually any short-acting drug, including alcohol and heroin, which also have the worst of withdrawal lasting 3-5 days in most cases, but with a PAWS (post-acute withdrawal syndrome) which can last weeks or months. Some issues, like impaired concentration, can last for years.