Why not?
When he surprisingly meets most of the average critereas in the OP which really are a benchmark used by Asian fans to rate their batsmen.
There is no one criteria for being an ATG. Averages alone don't tell the whole story; Younis cannot handle swing and seam, and throughout his career he has fished outside the off-stump like an amateur. He also hasn't played enough overseas, and it is not entirely his fault but this is how it is. Batsmen like Kohli and Rahane have played similar number of matches overseas, and they made their debuts a decade later. In 2004, he played well in Australia, in 2006, he played well in England and in 2006-07, he played well in South Africa, but Yousuf played better knocks. That has been the story of his performances overseas, almost every time someone from his team plays a better innings than him to outshine him. Yousuf did that in 2004 and 2006-07, Shafiq did that in Cape Town in 2013, and Azhar did that in Australia few months ago. England 2016 was perhaps the only time where his innings stood out compared to the rest.
In addition, he has weakness against bounce as well, but at times he has managed to counter that, for example the innings at Old Trafford in 2006. However, he has never produced a substantial innings against lateral movement, which is why he has not helped Pakistan win a series in any of the major non-Asian countries.
2016 was his best opportunity, with three consecutive series in England, New Zealand and Australia, and unlike the previous occasions, We had a pretty solid and settled team. In England, we had the good fortune of playing in late summer with good weather and grassless wickets, with Anderson and Stokes nursing injuries and Hales, Ballance and Vince trolling England in the top-order, but Younis took more than a month to adjust to the pace and bounce, and by that time, we were 2-1 down and a series win was out of question.
In NZ, the pitches resembled the Wimbledon grass courts and it was no surprise that he averaged 4. In Australia, he took two Tests to adjust to the pace and bounce, and by that time, we had lost the series. We played well in Brisbane and Melbourne without him playing any substantial innings, and had he showed up, we could have gone into the Sydney Test 1-1 or 1-0 up.
He has been a terrific player for Pakistan. A run machine who has made a great impact. In majority of the Test matches that we have won during the Misbah era, he has stood up for the team. However, he has not had a dominant series overseas which could have helped Pakistan win a series, and his failure rate of 82% last year (14 failures in 17 innings) meant that Pakistan lost 7 out of the 9 matches they played.
He is undoubtedly one of Pakistan's best ever, but he is not an ATG and he has not been able to place himself among the elites of the game. A league which has one Pakistani batsman only, and that is Miandad.