5, 6, 7 are no more "round" numbers than 180cm or 15cm or 10cm. And besides, the only reason you value "roundness" is because your system of measure has trained you to, because it becomes unwieldy when fractional units are involved.
Like saying “he is 6 feet tall” is slightly more ergonomic than saying “he is 1.83 meters tall” or “he is 182cm tall”. Both of the common metric versions of that measurement are less ergonomic than 6ft. cm feels too big and m feels too small.
Another way to put it is that the range of measurements for many everyday things often fits on your two hands.
Another example “I have a 7 inch penis” translated to, “I have a 17.78cm penis”, subjectively doesn’t feel the same in conversations.
The number 7 subjectively fits in your mind a little easier than 17.78.
Anyway, as I said I don’t disagree with you, the metric system is better in a lot of ways and I’m happy to use it when I can, but I do think that people spend far too much time talking about how superior their big brain unit of measurement is when it doesn’t really matter that much.
I can counter with that saying 180 cm is a lot easier than 5 feet 10.86 inches. No one says that, similar to that no one would say 17.78.
What’s convenient with metric is that there isn’t any difference between saying 180 or 179 cm. You just say, “one seventy nine”, not “one hundred and seventy nine”. Here I am showing what I am used to.
In your counter example, I would just round the imperial units to 5 feet 11 inches or 6 feet in everyday conversation. My point is that whether it's 180 vs 18 vs 1.8 all of those powers of ten are just a little bit less ergonomic because they do not fit nicely onto my two counting hands. 5 and 6 do fit nicely on my hands (and in my head) and so are just a tiny bit nicer to use every day.
To be honest, if the meter was closer to the size of a foot the metric system would have the same advantage. The powers of ten part is the good part of the system, not necessarily the absolute sizes of the base units.
I agree that if I grew up with it I would think it is fine. It is just what you're used to. I'm just expressing an opinion, I don't know why everyone needs to downvote me, I keep saying that metric is better.
Metrics allow to look into much large range inside the same system, which allows quick mental estimations; converting parts of an inch into feet or yards are less convenient. Then we can talk about areas and volumes, where metrics multiply nicely, but with imperials there's a whole set of special units.
When you talk about a 7-inch penis you're rounding off the length to the nearest unit. Or do you really have a 7.00 inch penis, rather than a 7.11 inch penis or a 6.93 inch penis?
If we're talking penis length in centimetres, you'd round off the number same way you'd round off the number of inches. So you wouldn't say "I've got a 17.78 cm penis", you'd say "I've got an 18 centimetre penis."
In the same way, instead of trying to buy a pound of meat in metric, you would not say "I want 453 grams of fillet", you would say "I'll have half a kilo of fillet, thanks." And just as the butcher would never weigh out exactly 1.00 pounds of meat, he's just as likely to weigh out half a kilo of meat plus or minus maybe 10 or 20 grams.