Perhaps they wanted to dive in all at once, saving it all for later.
Or it could be people that just like to collect the games.

Thats the issue  as games have grown and matured, their difficulty has dropped. it didn't grow with them. And by difficult I mean games that require a certain degree of skill. Eg. ninja garden series Dark souls, bloodborne....etc. Once upon a time, games like that were the norm, and when the game lacked a heavy skill based combat system they in its place had puzzles that would at least require you use your head. 

What we have today, that in most cases you call depth and content, is literally quantity over quality. I'm not saying games should all revert back to those six stage games of old. But I'm saying those games showed that you don't have to literally have 40hrs of content to have a good game. 

do you realize that instead of having a game lowered with 20 fetch quests and 20 go here and kill these enemies quests they could make only 10 proper side quests with real depth and substance and that would be enough? Ever play games like soul reaver? there wasn't a single side quest in that game yet if you ask anyone that has played it they will tell you it was one of the best games they played. 

And hard games shouldnt default to indie sidescrollers. Games these days are just too easy. Even uncharted on crushing is easy. And I see all that as a manifestation of continuously poor game design. There is a reason why game guides were so popular back then. 

take this for instance, in FF7 and example of a "side quest" was getting the gold chocobo. If you hsve any idea what I'm talking about and know what it entailed to do that then you will see just how far down game design has gone. In FF8 there was a certain something you would get if you could dodge the lightning strikes 99 times in thunder plains. In vagrant story the possible combinations in weapon crafting was a game in of itself. soul reaver took enviromental puzzles to a while new level. I could go on and on. 

I'll just settle for this instead; games are poorly designed now. Aiming for quantity and glamour instead of engaging challenges. 

I buy a lot of games. Sometimes I just want to see what they look like before i put them away for later. I have a few games with just minutes of play. That backlog is no joke.

On the Witcher a lot of achievements/trophies are missable.

Personally, I have all notifications turned off on my ps4, I hate hate hate trophies, they ruin my games.
on PS3 you had to "synchronise" your games, so I have hundreds of games which I played but (luckely) don't show in a list.

I hate achievements and trophies because they force you to play a game in a way thats not my own.

Anyway, thats might be the reason for some of the "rare" "missable" stuff beintg rare, because people don't care.
But in many games the first achievement or trophy, which you get in like the first hour of the game is also only obtained by 50% of the people, and this also doesn't make much sense to me, but apparently lots of people buy games and never play them.

I guess  I have about 10 games on Xbox360 I never played and about 4 on PS4, and I did install them so they will show up as 0%

1. People that played the game but never owned it are included in these numbers. Let's say you have multiple accounts on your Xbox (girlfriend, brother, cousin, other friends etc.) and you/they start the game because they want to see what the game's like. Obviously they're not going to finish it, but that's still counted towards these numbers. This can also easily happen as an accident, when you're still logged in with another account and then start the game, thinking you're signed in with your own.
20% for the completion of such a long game is really not bad.

2. The first story based achievement (for finding Yennefer of Vengerberg) isn't unlocked within the first hour of the game. Actually I can't even tell when exactly, but I can tell you that I bought the game maybe 2 weeks ago and started it, played it for 2 hours and haven't earned that achievement either, because I did some side stuff first. So I will probably unlock that achievement after... say 3 hours.
Now keep in mind what I said under 1. because many people that only tried the game at a friends' console and didn't buy the game afterwards probably won't play it for more than 3 hours.

3. The achievement is known to be a little glitchy. The majority of players didn't have that issue of course, but it will happen to some and thus reduce the percentage a little bit.
https://www.trueachievements.com/viewcomment.aspx?commentid=879645

isnt there a way to hide your achievement trophies? i doubt 35% of the people who have bought it on xbox didnt even finish the first mission

i honestly wouldnt know.

but i think barozi gave a few strong reasons.

After reading more posts in this thread I get the feeling that many users don't seem to grasp that these achievements we're talking about here are story based and unlock when you reach a certain point in the game.
You don't have to do anything special to get these, so saying I'm not playing for trophies/achievements is completely irrelevant. You're getting the achievements/trophies in any case, if you want them or not.

Them being glitched would be the only way not to get them.

Doesn't have anything to do with skill. I also didn't complete The Witcher 3. Not because it's too hard (it's easy as fuck) but because I was bored after a few hours and never touched the game again.

I beat Zico in Wipeout HD, I got the plat in Killzone 2 and I don't mind playing friggin' hardcore oldschool games like Contra or R-Type. I actually love a decent challenge. No, this isn't about skill. The Witcher is just a boring game, period. =P

Insert lyrics to "That's Life" by Sinatra.

You can turn them off on both consoles if you wish.