This question already has answers here:

Using an Alias column in the where clause in Postgresql (6 answers)
How to use row_number in a where clause (1 answer)
Closed yesterday.
I can get the rank alias with this query:

SELECT *, 
  RANK() OVER (PARTITION BY some_field ORDER BY value) AS rk
FROM my_table

Result:
| some_field | value | rk |
| ------------ | ------------ | ------------ |
| same | 10 | 1 |
| same | 20 | 2 |
| same | 30 | 3 |
And I tried to query with:

SELECT *, 
  RANK() OVER (PARTITION BY some_field ORDER BY value) AS rk
FROM my_table
WHERE rk = 1

I got this error message column "rk" does not exist
If I tried a subquery, it works :

SELECT *
FROM (
  SELECT *,
    RANK() OVER (PARTITION BY some_field ORDER BY value) AS rk
  FROM my_table
) AS t
WHERE rk = 1

Result:
| some_field | value | rk |
| ------------ | ------------ | ------------ |
| same | 10 | 1 |
But my question is why can't we use just one SELECT to do so.
Is it because I use a function in my query?