Why mashed potatoes?

Gravy.

Canadian as well
We do roasted veg (including spuds) and mashed and it’s all because of the gravy!!!!!!!

Hmm, not sure. I know that most recipes for roasted winter vegetables that I see don't include regular white fleshed potatoes. A traditional Yankee pot roast (usually beef) is paired with white potatoes, carrots and onions, cooked alongside the meat (which is usually braised rather than roasted), but that's not Thanksgiving food. When I serve roasted vegetables at Thanksgiving, it's more likely to be squash, brussels sprouts, peppers, etc. rather than white potatoes. Mashed white potatoes would be in addition to that.

Maybe it is just the opportunity for gravy.

We had a smoked brisket instead of a turkey this year anyway, and I served mashed potatoes with that even though it's far from traditional with that kind of meat. *shrug* We eat what we like, I guess.

It is, of course, late fall here in November (as opposed to the late spring you're currently experiencing), and traditionally, fresh vegetables would have been limited to root veg at this time of year. The more potatoes, the better, I'm thinking.

Mashed potatoes can be prepared on the stovetop (boil the potatoes) while the turkey's in the oven.

I'm 72 years old and have had mashed potatoes and gravy every Thanksgiving that I can remember. It isn't even something I question. On thinking about it, my oven is full of a huge turkey. Potatoes can be boiled on top of the stove and made so good with cream cheese, butter, milk, sour cream and lots of pepper when you mash it. That's heaven on my tongue so why mess with a good thing.

Adding gravy and stuffing to that bite of turkey and mashed potatoes...that's how you make it better!!!

But really, they are cheap and easy to make (either while the turkey is cooking or ahead of time) and the other reason would probably be tradition.

Also, mashed potatoes aren’t the only side.

We have Brussels sprouts, some type of corn, dressing, squash, green bean casserole, Mac and cheese, rolls, salad, a cheese tray, and sweet potatoes (with marshmallows on top of course). Oh and cranberry sauce.

Definitely because mashed potatoes go so well with gravy.
Our Thanksgiving is in October, and we usually have steamed broccoli and cauliflower, baked sweet potatoes (no extra sugar needed), sometimes boiled and mashed squash.

We eat a lot of roasted, baked, and oven-fried potatoes year round, so real (not instant!) mashed potatoes are a treat!

If I roast a chicken or beef I will sometimes roast potatoes, carrots and onion in the same roasting pan but when roasting a turkey, mashed potatoes are the traditional side for Thanksgiving dinner in Canada and the US.
camcas When do you usually serve mashed potatoes?

Another Aussie and I've often wondered this as well. I think the reason it seems unusual to us is because mashed potatoes here is more likely to be served with a cheaper weekday meal. We don't tend to have them on special occasions. Roasted (not baked), hasselback or some other form of potato preparation is more likely to be seen for Christmas (we don't do Thanksgiving obviously). And we'd have the gravy regardless! Gravy to me belongs with the meat. I'd never have any sort of roast meat without gravy! Just one of the many small cultural differences between our countries that make life interesting I guess!

And for whoever mentioned "instant" mashed potatoes - what on earth is that? Surely mashed potatoes are always made with fresh potatoes, aren't they?

I think there's a difference in terminology. How would you describe your method of cooking a turkey? If a piece of meat is cooked in the oven in a baking dish/tray with some form of fat in the bottom we would call it a roast. We would probably have mash with sausages or fried chicken or a casserole.

lol Sounds like we are understanding what a roast is with it being the method of cooking, therefore a roast.  A thought just occurred to me, maybe in America, they don't roast the veggies with the turkey because it seems like there was a thing of bigger is better. No room for veggies when you picked out the biggest turkey imaginable to up one your neighbor, therefore, cooking the veggies in other manners.

I've always thought the more puzzling thing is - since when is pumpkin a dessert ingredient and marshmallows something to add to a vegetable dish?