You can’t have water/moisture getting in without air getting in as well. Got to make your mind up. Its either sealed or not 😉
And it gets in with the dirt that gets in through the seals, it gets in through the reservoir (some comes back out out, not all), gets in through the hose connectors being wiggled and moved, or the banjos being moved/adjusted/knocked, the pistons being reset etc etc.
Yes, not all will get air in, it depends how they are used, fitted and cared for.
And FWIW the brakes on our race bikes used to get bled once a year, mine would always come out cleanish, just a little bit of discolouration in the stuff at the caliper end on the front. My other halves front brake would have a slug of black oil in the caliper. Despite doing the same races and very similar training.
Funnily enough, when the brakes were all swapped round, the black oil *still* ended up coming out of my other halves front brake, even though she had my rear brake caliper………
And for the sake of an extra 20 minutes and 10 pence worth of oil per bike once a year when the bike is up on a stand being “dealt” with, i’ll carry on bleeding brakes yearly…… (well, not on my hack bike, i’ll keep that going until it breaks.)