It's extremely tough.
While individual legs aren't all super complicated, the facts that you're operating on much less oxygen in your blood, way below freezing, with gear that weighs you down, in clothes that encumber you, and on little sleep after weeks of getting there, conspire to make an Everest summit super hard.
On paper it's easy. For about a week you'll walk and climb about six hours every day. And then you're at the summit, look down, take the five minutes or so that you're allowed before you have to get down, and then it's all peaches and cream...
Well, not really. First, your first day on the summit is across the Khumbu icefall. Not only is this a super dangerous place, it's also extremely exhausting. People die here. Lots of people. It's essentially a frozen waterfall that still moves slowly.
It doesn't get much easier from here. You'll sleep (or, likely, try to) in the hopes that around midnight or a little later you'll get a shot at the next leg. You'll abort legs. You'll deal with altitude sickness, hunger, sleep deprivation, frostbite, and your eyes will sing the song of their snow-blind people.
Until you're on the summit you'll never know if you make it. I only tried once and we had to abort before the Hillary Step because the weather got super bad. Luckily we did, because the coming blizzard would have killed everyone. People die not just from the weather. The same year a climber from the UK died after going blind on the descent and having had to be left on the mountain by his team who would have died as well, had they tried to get him further down.
Once you're up there, you have two or three chances to make it. No one operates well at that altitude; you have to come down eventually. People like(the much maligned guide but actual hero of the 1996 expedition, not ) have operated up there for more time than actually humanly possible without oxygen or sleep, but even for super trained mountain men it's a question of one wrong step (Boukreev died a year later from such a mistake).
More people die on the way down than up. Many simply exhaust themselves and get careless or just collapse on the way down. Others are being held up on bottlenecks (Everest is big tourism, now) and are already fully spent by the time the hardest part, getting back down, comes, through no fault of their own. Then there are the ones who won't abort and think they can make it.
That all said, loss of limbs, life, and loved ones aside, compared to other ascents, Everest is actually not that hard. It's big tourism, so there are facilities for everything. If you're super rich you can have two to six porters haul you up there. If you're Larry Page rich, you don't even have to walk to Base Camp and can get a helicopter to that place, your own tent with heating and fresh food, your own doctor, and people who carry even your fixed lines for you.
Annapurna (48% fatality rate), K2 (23% fatality rate),(18%), (29%), Denali, even the are harder mountains to summit and have a higher death count. Partly because they're much less supported by climbing tourism, partly because they're steeper and have even less predictable weather. In the case of Matterhorn it's (like Everest) its popularity, with climbers kicking off avalanches and loose rocks onto climbers below.
The deadliest of the top 50 summits is, by the way, Mont Blanc. Not in statistical likelihood, all the above trump it, but in sheer numbers. Over 8,000 deaths have occurred there, the most summit deaths (400 for Everest without the earthquake, 300 for Nanga Parbat) in the world.
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