Being careful helps, but various airlines and countries have different regulations about which luggage ammo MUST be kept in, and often it must ALL be in a single, locked box. Some require it be in the case with the firearm; others don't allow that at all.
In once instance, the hunter departed for Africa on an airline that didn't allow ammo in the gun case. Everything was fine when landing in Johannesburg, but during a flight change IN SOUTH AFRICA, another airline required the ammo to be in the case with the rifle. This was a recent change nobody knew about, including the very experienced travel agent the hunter used. When the hunter's luggage was run through an X-ray machine the ammo showed up, and was confiscated by the airline, which did NOT inform the hunter. He arrived for his hunt with everything he'd packed except his ammo.
In another instance, only the hunter's rifle case showed up on a Canadian hunt--and his ammo (per the airline requirements) was in his other luggage. His rifle was chambered for a wildcat .338 caliber, and .340 Weatherby ammo MIGHT have fit the chamber, but was never tried, because there wasn't any .340 Weatherby in any sporting goods store around.
I even know one guy whose ammo case was stolen during a vehicle break-in during the drive from the airport to the hunting lodge, probably inadvertently by some thief who just grabbed whatever he could easily carry, because the heavy rifle case wasn't taken.
Those are just a few examples, but lot of it just simple odds: The more somebody travels to hunt, the more likely ammo will be separated from the rifle. I don't travel as much as some people (one friend has been on 50+ African safaris, and hunted in almost 100 countries), but as noted previously, even in my more limited travel I've lost count of companions like the three guys just mentioned.
Despite that, I've traveled several times with rifles chambered for relatively unavailable cartridges, at least where the hunts took place, such as the .358 Winchester and 9.3x62 Mauser. (In much of Africa 9.3x62 ammo can be found in many stores, but not in Alaska and Canada.) Once I even brought a rifle for a real wildcat--not just an "improved" version of a common cartridge, where factory ammo would work. But in each of those instances I either took along another rifle in a common chambering, or made sure a rifle could be borrowed, either from a traveling companion or the outfitter. And when I've brought a spare rifle, it's ended up being borrowed more than once, because a companion's ammo or rifle didn't show up.
So you've never known anybody whose ammo got separated from their rifle on a trip. This just indicates you and your friends/acquaintances don't travel all that much. While the odds are with somebody who doesn't travel much, that doesn't mean it can't happen on their very first trip--and no, it usually isn't their fault.
“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.” John Steinbeck
