Let’s get one thing out of the way: it’s not normal to stuff $100,000 in cash into a suitcase and then check said suitcase on to a domestic flight. It’s extremely weird, in fact, not something you’d expect a typical person to do in their lifetime. However, doing so is emphatically
You do realizing that explaining why will not cause them to let you keep the $100k. They WILL seize it, regardless of your reasons. They take note of those reasons you give so they can use that against you in a court of law, however.