How many sets do you buy per year, out of curiosity? I got into Lego during the pandemic and bought around 50 sets in the past three years. Over that sample, the number of missing pieces was precisely zero.
I have never had a missing piece. There have been two times when I thought I was missing a piece and then found it stuck in a bag or hidden under another piece. I've never had to write to Lego to get a replacement (which I've heard they are really easy to work with if it does happen).
Out of curiosity, I just texted my friend to get another sample from him and he said it happened to him one time, but also admitted his kids might have been to blame.
So when people claim that every other set they buy has missing pieces, I always feel like I am either the luckiest person to ever live, or maybe the pieces are there and certain people are just more likely to misplace or lose them in the building process.
Both occasions the pieces weren't structurally important, and were small decorative elements.
I've yet to find a single missing piece so far. Many times we thought a piece would be missing, only to find it trying to escape somewhere. The plastic bags carry a pretty heavy static charge in dry winter conditions, and the tiny pieces love to stick inside those in the corners where they somehow turn damn near invisible.
To be honest the quality control is pretty unbelievable. If someone tells me they had 6 sets with missing pieces over the past 5 years my initial reaction would simply be to not believe them as it's so easy to misplace a piece during a 2,000+ piece week-long build. I've misplaced dozens to the point of having to order replacements from ebay or whatnot - only to find the pieces lurking around my house before the new parts even arrive. The joke of the household is you need to order a replacement part and then you'll find what you're missing a few hours later by complete accident.
I've found pieces in the strangest places. If you accidentally sit on one they will basically become one with your flesh, and I've found random pieces floating in the tub completely unexpectedly.
Knocking on wood my luck (and Legos QC) continues.
But in the dark they become savage. Stand on one or even worse, kneel on one when trying to find the tv remote. I might have cried.
And I've had to return a set or two to Amazon because they'd clearly been opened and returned (and usually all the minifigs stripped out).
In other words, I would not expect the distribution of Lego sets that are missing pieces to be evenly or randomly distributed.
That is why you always get two helmet visors for example when you buy a speed champion. The machine is actually just designed to give you two, because that way if it breaks and misses one, you still have one that you need and it costs them almost nothing to do that. But a missing helmet visor is easy to get missed by the quality control scale. So if you ever buy a speed champion and only get one helmet visor, it is because you lost one or the machine broke. But that's why there's an extra. They do that by design. That's just one example. You see it with magic wands in Harry Potter sets, you always get two, and a lot of sets with 1x1 pieces will always have 1-2 extras just because they are cheap to add in and more likely to get missed in their QA process which weighs the bags.
I think that’s calculated in their cost of doing business. Add one extra of each 1x1 probably eliminated most of the missing parts issue at a fraction of the cost.
Almost every Lego set I've ever owned (going back 30+ years to when I used to get Lego) and now my son's Lego recently have all had at least one leftover piece.
The smaller sets have all been fine, tho.
(Also zero shelf space left to display any more Lego...)