Because we learn?
Because they're available commercially so don'tso don't be homemade? There was some demand though, I have a couple of knitting and crochet patterns from the 70s for greyhound type jumpers.
My grandparents wouldn't have put coats on their dogs.
They also fed them table scraps including food we now know is toxic, left them outside all day, would have assumed crates were used for cruel punishment, didn't take them to the vets for preventive medication/vaccinations, and thought 7 years was 'good going' for a dog.
My dog doesn't like wind in her ears or being rained on. Neither do I, so I don't think it's unreasonable for her to wear a snood over her ears in the wind, and a raincoat (with hi vis stripes) when it's wet.
I also have shoes for her for icy weather, mostly to avoid road salt and other toxic substances from sticking to her paws and then being ingested. Given the amount of broken glass around, I'm starting to wonder if she needs them on every time we walk on pavements, though for now we're sticking with grassed areas instead.
She's in her late teens now, a bit blind, mostly deaf, and likes her comfort. She was 'stealing' my oodie on cold mornings, so I treated her to her own. Mostly so I could reclaim mine guilt free.
And probably to the horror of some of the commentators on here, my dog has socks and likes wearing them.
It started as a gag-gift from a secret Santa, I put them on expecting that they wouldn't stay on long enough for me to take a photo for the gift giver, and entirely unprompted, when the first one came off, my dog brought it back to me to put it back on. She knows where they're stored, and on cold/wet days, will sit next to the shelf, looking at them and back at me.
Anyone who says dogs can't talk, isn't paying attention.