The pros know what they are doing. Because they are pros. Thinking one knows how to design a knife just because one has used a knife is like saying you can design a car engine because you know how to drive.
Full disclosure: I'm a Yahoo from Instagram and YouTube, and the Yahoo from OP's post. But you've got a great point here, and one that I try to be *very* sensitive to during reviews and stealth reviews. When a maker or company sends something my way for a stealth review, I try to be very sensitive to the fact that, well, in your words, I can't design a car engine. I'd have to be delusional to pretend I can guide them in "design" writ large ("this would look way better if..."), or to tell them how they "should" make the knife. Heck, some of the folks I talk to have forgotten more about knife design than I'll ever know.
That's why I try to focus on providing constructive and concrete feedback for the maker about individual issues, rather than critiquing the entire knife or the aesthetic joy of it. Sometimes this feedback is in the form of "Wow, this is very different from the current market because [blah], I love it!", or "Looking at the field, your price of [blah] is probably gonna face some pushback". Most often, this takes the form of spotting little functional fouls that I've started to notice after critically handling a large number of knives, like the ones I discuss in my knife-gripes series. Given the huge number of variables that a maker is juggling, it's not shocking that one or two of the "picky little details" I tend to focus on might slip through, even for a great design.
Sometimes, makers agree and make changes. (Often it's sort of a "Oh, whoops. I missed that [thing]!") But just as often, the maker points out why they can't avoid an issue or issues, or they tell me that they prefer it as is, which is fine too, because it's their knife. if they decide an issue is worth tweaking, they have a chance to before it goes to production.
Anyways, I don't pretend to be a designer. I'm just a random jackass who's handled and used a lot of knives, has noticed a lot of recurring issues, and has a decent view of the current market. Some makers have found my feedback on a new design helpful, which brings me great joy, and in many cases, I'd like to think it's resulted in a bit less ugly in the knife design world. Some, I imagine, don't give a damn what I think, which is A-OK, too!
But regardless, I'm doing my best not to pretend I'm anything I'm not, because the last thing I want is to wind up designing the Homer!