Is it as important that kids clothing fits precisely though? I mean, evidently it is to you but I would I would have thought parents would be happy that the kids look decent enough. You even point out their rapid growth so it's not like they're going to be continuously wearing the perfect fit all the time. Buy slightly big and they grow in to it.

It's not so much "precisely" as "not ridiculously wrong". The difference between "slightly big" and "tent" isn't much, considering there is practically no objective basis to kids' sizings, which are basically the intended age with no consideration of variations in age & growth; the sizes do not reflect an actual size.

Yes, this! Just to get close to fitting, we have to get our son pants for a 6yo, shirts for a 4yo, and socks for a 3yo, and even then we end up rolling the sleeves on his shirts because he needs the length in the chest but not as much the arms. I can't tell you how many pants, shirts, socks, etc. we went through before figuring out this is the combination that works. Oh, yeah, and this is only with the specific combination of pant/shirt/sock brands that he's currently wearing. If we had to replace one part with a different brand (which we will, because he's growing and for some reason there's no brand that reliably stocks the full toddler-kid-youth size range), we'd be screwed.

Most important (for us, anyway) is t-shirt quality and softness. Like you said, we buy big and they grow into it and out of it but it's the quality that really matters. We buy mostly Hanna Andersson shirts for our boys. They're expensive but they still look good after two seasons of wear, which is remarkable for kids' clothing. Our younger boy wears many of the HA shirts and coats that his big brother once wore.

This applies more to shoes than clothes. It's ok to have a little bit of variance in clothing, but given how much kids run around, they need shoes/sandals that fit really well.