Because they don't mean anything. All they mean is you managed to not shoot yourself for at least 2 days, mostly. Most instructors don't have a "test" to even "pass" you from the class. Tom Givens does an exam that you have to do until you pass it. I've seen someone have to shoot it 4 times and that was the worst. I don't know if he ever gave up on someone and didn't give them the cert. Other classes I took, the certs were given out when the day was over and no requirement other than "not getting kicked out"It's just me but I don't quite understand why certificates don't matter. To me a few would make your instruction highly plausible.
And like you said, you've trained with Nate. Well. Who knows who Nate is? If I go to some of the local instructors in Missouri where I'm at, I don't know if they know him. I don't know if he knows Dave/Don/Chris of Milcopp, or the guys at this new place in Farmington, MO. So to either of them, the others' certs may as well be a diploma from Hamburger University.
They don't show what topics were covered - your ability to grasp them, or anything of the sort. The closest you can get is if you KNOW the instructor, KNOW the curriculum of the class specified on the cert, and even then you still don't KNOW how the student actually did, or how safe they were.
I keep my certs because "you never know" but they mostly just sit there in a folder on my gun parts shelf in the man-cave, where some other gun-related paperwork is kept.