It's because they are male, obviously.

"Man" is the word we use to describe an adult person who is male.

Ah but what if I don’t subscribe to your belief system?

‘Man’ js the word tradition taught you to describe an adult person who is male - but what does that mean, really? Is it a person with a penis? If a man’s penis is removed, what is he then? Prefer his genitals at birth, maybe? What if he’s born without a penis? Intersex people exist. Chromosomes perhaps? There are all sorts of extant combinations beyond XX and XY.

The real question you should be asking is, why does it matter? how does this belief in men and women serve you? It seems to me like your insistence in following this tradition is actually hurting you, not helping you, because it’s narrowing your understanding of your fellow humans, to the point where you can confidently say things like “it’s because they’re male obviously” as if that doesn’t make you look incredibly foolish by modern standards.

At the very least, you may want to consider keeping your belief in gender mythology to yourself.

"We" is an assumed group of people who agree with you.

The alternatives - that "man" and "woman" are identities that anyone of either sex can claim, or that "man" and "woman" are defined by a narrow set of cultural stereotypes - are very niche definitions that should be disregarded as, respectively, absurd and sexist.