Understanding Oracle aliasing - why isn't an alias not recognized in a query unless wrapped in a second query?

I have a query

SELECT COUNT(*) AS "CNT",
       imei
FROM   devices  

which executes just fine. I want to further restrict the query with a WHERE statement. The (humanly) logical next step is to modify the query followingly:

SELECT COUNT(*) AS "CNT",
       imei
FROM   devices
WHERE  CNT > 1 

However, this results in a error message ORA-00904: "CNT": invalid identifier. For some reason, wrapping the query in another query produces the desired result:

SELECT *
FROM   (SELECT COUNT(*) AS "CNT",
               imei
        FROM   devices
        GROUP  BY imei)
WHERE  CNT > 1  

Why does Oracle not recognize the alias "CNT" in the second query?