Why don't user variables work in where?

Question

SQL

SELECT
  `num`,
  @tl:=`num`+10
FROM
  (SELECT 1 as `num` UNION SELECT 2) as `t`
WHERE
  @t1 = '11'

Why don't user variables work in where?
At the same time, they don't work in having either...

Answer 1

SELECT
  t.num,
  t.num+10 AS tl
FROM
  (SELECT 1 as num UNION SELECT 2 as num) as `t`
having
  tl = '11'

Answer 2

Paka Declare your variables