I think the same as you that Milan should have won this match, but Atalanta defense can't be considered weak, sharing points is not bad either. Unfortunately Junior Messias shoot the ball in the 23rd minute sideways, if it make a in score maybe we can see Milan won, even though the breakthrough at that time had made me get up from my seat, not as expected.
If Milan had won this match, it would have been unfair to Atalanta, who clearly played no worse than the reigning champions and at least the club was worthy of a draw. It is difficult to draw far-reaching conclusions yet, but so far Milan does not look like an absolute favorite and the club still needs to add to the game. Other favorites, for example, in the face of Inter and Napoli, still look brighter.
Fairly balanced game I think and this draw is fair for both teams. Both Atalanta and AC Milan were able to perform well, even though AC Milan dominated, but Atalanta was able to compensate and provide difficulties for AC Milan. From the beginning I also believed that this match would most likely end in a draw.