The US actually does have a lot of Indian restaurants (and ethnic and vegetarian places in general), especially near places with more population/diversity/Democrats (which all kinda go hand-in-hand here). Indian, Thai, Chinese and Japanese restaurants are in basically every strip mall (what we call a bunch of random small retail businesses sharing the same parking lot) and every downtown.

However, even then, you're usually stuck with the same few options everywhere you go. At Indian restaurants you usually do have a bit more variety (thank you Hindis), but it's still the same like 9-10 entries... your typical chana masala, palak/matar paneer, aloo gobhi, pakora, biryani, etc. And if you're vegan, it's either the chana or the daal if you want any protein at all.

At other ethnic places, it's typically more like 2-3 options if you're lucky: Some sort of mixed veggie tofu stir fry at the Chinese places, pad thai/kee mao without fish/oyster sauce at the Thai places, marinara/lasagna/eggplant at the Italian places, falafels at the Mediterranean places, etc. Even the specialist vegetarian/vegan places typically have very similar menus, usually fried comfort foods or various mock meats, with some sort of nutritional yeast & cashew sauce, topped with tofu, seitan or jackfruit.

Compare that to the dozens of options a regular person would have at any restaurant, and the hundreds of restaurants in any city. Even with all the spices in the world, there's only so much you can do with legumes, grains, nuts, and leaves. You really do lose out on a lot of variety. Meats, milks, and eggs have taste and textures all their own, produced by the different animal species of the world. Only recently have we been able to replicate some of their tastes and textures in the lab from veggie ingredients, but it seems like that fad has mostly passed (Beyond and Impossible aren't doing so well, and the smaller brands remain niche, and omnivore interest around them has mostly waned.)

I say that as someone who's been vegetarian and then vegan in the US for nearly 20 years. Yes, there's more to eat than most people think, but still way, WAY less than any omnivore or pescatarian would have access to. After all, they can eat all the same things too, but add whatever animal ingredients they want on top of it.

The situation might be a little better if you're a good cook and can make delicious meals at home, but even then, you're fundamentally limited to trying to dress up generally bland plants & legumes with fancy sauces. And many cookbooks don't really provide balanced meals (typically lacking in satiety or protein), especially when vegan.

Overall, it's just a lot of time, effort, and expense, and greatly limits your dining options and also your social options (eating out with friends, going over to their place, hosting them for dinner, etc.). Hasn't really been a big problem since it kinda ends up being self-selecting anyway (the vegetarianism has good overlap with people who also share my values, hobbies, love of the outdoors, etc.), but that's a regional cultural thing. In more conservative/traditional areas, it would be absolute social suicide. Food is huge in many cultures and subcultures, and disavowing 90% of what people eat is a good way to get yourself shunned and mocked.