I think it is partly because combo guards are often tweeners and although they are valuable to have on your bench they are not ideal as starters. Ant is best as a shooting guard but his 181 lb frame is not ideal to guard bigger SGs. His length is fine with a 6'9" wing span.
Fans also have a tough time separating a player's salary from his production. And because fans think there are plenty of 2-way players out there that would be better....when there really aren't.
For me, it comes down to who he is surrounded by. For example, let's say we had a point forward who was a great defender (i.e. Ben Simmons), and let's say Shae improved his defense and played SG...... and we had a SF who was also a great defender and could slash to the basket, and we had a center who was a great rim protector........Ant would be a great guard to have on the court with those guys. Not so much next to Dame, Hart, and Grant.
With Scoop as our PG, I think Ant is best as a 6th man.....forget about his salary. If Dame is gone, Ant becomes the best outside shooter on the team....is that even debatable? That is not good enough for some fans but it is still a nice skill set to have at the end of games when we have to score.