How are Indians not considered “Asians in practice” when they are considered as such by the Census, the authoritative organization that sets the categorization standard?
Because laypeople don't think of those things. The divide in the way people look, the environment of their countries, how they sound, and what they believe, is split in Asia by the Himalayas. Obviously there are many differences between the countries but it's easy to see why conceptually some might group some of them together into 2 large regions. Also Indians in America don't generally refer to themselves as Asians when speaking, so if their goal is to be perceived as Asian, they are not helping themselves.
Colloquially, in America, Asians are identified as having slanty eyes and white skin. Indians have brown skin and are therefore not colloquially referred to as Asians. Instead, they are referred to simply as "Indian".
This is an oversimplification, most Americans will also be quick to categorize darker skinned Southeast Asians as Asian.
Historically most Asians in the US were from East Asia and later SEA so the label tends to evoke those groups. Lacking familiarity, it may also be harder for many Americans to easily discriminate (lol?) between East Indians and other non-white/non-Asian groups.
Still there are plenty of people that do include Indians as "Asian" in the US, it's hardly a universal standard to exclude them as a lot of comments here seem to think.
In my experience living in the U.S. at least, if someone is referring to someone of Indian origin, they call them Indian, whereas if someone is referring to someone of Korean/Japanese/Chinese origin, they'll call them Asian (unless they know their particular country of origin).
I would expand the Asian designation to include southeast Asians as well (Vietnam, Philippines, Cambodia, Thailand, etc). Though maybe that’s just a west coast thing.
The GP is correct. Of course officially Asia has many nations. But colloquially we use 'Asian' to refer to East Asian folks. You're talking about a country that, in many parts, refers to Native Americans as 'Indians' to this day...