The UK is ahead when it comes to catering for various diets, because a fairly large percentage of the population is of an Indian/other south Asian background, where a vegetarian diet is much more common. The US has a similar demographic but, to put it bluntly, doesn’t care much about catering for them, with the exception of big cities and the coasts.
I’ve been vegetarian since birth and grew up in a Southern European country where it was virtually unheard of - like a PP said, everything that was naturally veggie came covered in bacon or tuna. Eating out was a nightmare.
Whenever we’d travel to the UK, it was like being in paradise. You had a choice of vegetarian meals on menus. I tasted my first burger at a McDonald’s in London. Not fake meat, chickpea and lentil. I didn’t even know that was a thing. If you look at the UK in general, they always have a veggie option on the menu, including at school. On top of veggie south Asian children, there are those who follow a Halal diet, and many prefer to choose the safe veggie option. Back in my hometown, we have a large percentage of children whose parents come from Morocco, and it’s been an ongoing battle with the council to offer them an alternative to eg seafood at school - seafood is served most days at lunch. They simply don’t care, it’s one meal for everyone and it doesn’t matter if these kids can’t eat it.
Now back to the US, it is true that it’s easier finding veggie options on the coasts, big cities in general, and university towns. These tend to cater for a much more international and younger demographic, so you can find many different world cuisines and vegan/veggie or GF/dairy free alternatives.
At the end of the day, even in the US there will always be something veggie you can eat, but it will not necessarily be a substantial option. I remember going to a 4th of July country fair in the Midwest, where all I could eat was corn on the hob and a variety of deep-fried sweet pastries. Don’t get me wrong, I love corn on the hob, but when that’s the only thing you can eat for the whole day - whilst everyone is having sit down meals with 3 courses and numerous choices - you do feel uncatered for. Of course it was not unreasonable of them to provide such food, because 99% of the people visiting the fair were locals, who ate meat, so why would they bother with lots of alternatives which almost no one would’ve chosen.