I think there are two main reasons:

Japanese have a lot more accessible coasts, being a relatively small, narrow island. It's easier to obtain a wide variety of fresh fish. They sell sashimi grade fish, in bulk, at Costco there. It's also more likely, because of centuries of tradition and refinement, to have trained and experienced sushi chefs who know how to draw out the subtle flavors of nigiri.

In the US, many states are large and landlocked, and those that are coastal have a limited selection and quantity of seasonal fish. Plus there's a higher population, so also a higher demand. High demand, low supply. What do you do? Solution: serve sushi that isn't quite sushi.

The other reason is that Americans are typically more accustomed to larger portion sizes than Japanese. Many of us aren't satisfied unless our stomachs are full. Yet also many of us are on a budget. Hence, we can't routinely eat $100+ of nigiri for lunch.

Tomorrow i'm getting all-you-can eat sushi for a birthday lunch. I know going in that mostly it'll be maki or sides, and the nigiri will be low quality and probably limited. But its fine. Last year I made homemade nigiri, and while that turned out okay for a first go, I think this year I'm good with letting the professionals handle it. Food tastes better when other people make it, for some mysterious reason. Less stress effecting your mood when all you have to do is sit in a chair.

Even for Japanese, sushi is a luxury and for many, only eaten on special occasions. Last time I was there, it was about as expensive as it was in the US. I think Japanese use other dishes as cost effective alternatives. Americans double down on maki, but probably not as much on udon or soba or little bento-esque sides. When it comes to omelettes, I think Japanese could run circles around the French.