How do I explain to my dad, using simple language, why "it's just a secret military aircraft" doesn't fully explain what people are reporting when it comes to UFOs?

Whenever the subject comes up, that's his go to answer. I understand why it's appealing, it feels like the grounded, no nonsense explanation and doesn't ask you to believe anything unusual. But when you actually look at the reported cases, it starts to fall apart: flight characteristics that don't match anything humans have built, sightings dating back well before we'd have had the technology, and similar reports coming from multiple countries independently of each other.

I'm not trying to push him toward any specific theory, I just want a clear and simple way to show him why "secret military project" isn't a complete answer on its own. If anyone has a good way to explain this without sounding condescending or getting overly technical, I'd appreciate it.