insert different from union?

I consider the SQL "insert" statement. It doesn't look like a primitive to me; it seems like a special case of the union operator.

But maybe not:

create table test (id int primary key);
insert into test values (1);
insert into test values (1);

If the insert statement was really the same thing as a union, then the second insert would be no problem. Instead, I get:

  ERROR: duplicate key violates unique constraint "test_pkey"

Now, if I had a table with a primary key and other attributes, and I inserted something with the primary key the same as an existing row and some/all of the additional attributes different, then I would expect an error. But in this case I get an error even though there's no incompatibility with union.