Have you ever tried to knit a dog sweater? I have! Here's my attempt and the pattern I used ($2 USD). Even with all the time spent and money on yarn it wasn't perfect and stretched out with use. Eventually I went looking for the best knitted dog sweater and found only one that was still being made by hand, and of course fitted the best!
Something missing from most dog sweaters is a fit that goes fully around the belly and hugs the curves, especially to that of a dog with deep chest. Usually the stomach panel is a thin rectangle that allows the sweater to rotate as it's loose and doesn't keep the sweater upright.
Ribbing keeps the dog sweater able to stretch yet spring back into shape with use. Every sweater I've seen mass-made for dogs do not use the extra yarn for large thick ribbing. The only one is Bravehound. It's very expensive to knit, yet so beneficial for long-term wear. You can see the large ribbing going all the way up to the cableknit back.
Here you can see a poorly fitted sweater, but what the heck, it's for Halloween! The back panel on the puppy is just a square that will flip over but she'll outgrow it in 2 weeks. However hand-knitted fits like a glove and hugs the curves like nothing else
Chest and arms that are not a mere hole in a rectangle shape are rare. You can see this sweater is formed from two rectangles with a hole for the arms. The stomach panel is stretching and loose in parts and the arms stretch the back and squeeze the top of the arms.
This sweater is shaped to a dog's form and accounts for a dog's chest shape. The arms are full circles (not slits) and are ribbed throughout to account for movement and stretch. It's not possible to lay the sweater down with the top showing like the plaid one as the Bravehound sweater is curved and shaped.