Because that's exactly what he should be doing in midfield two. Why is that weird? The same criticism is aimed towards Fred and McTominay.
It's just very difficult role to play, especially in current United side.
Kante is a box to box player, very strong defensively but he also has a drive forward. I don't think it's beneficial to make him pure defensively focused player, as last season for Chelsea shown how much he can offer in general play.
Also, if Kante can't do it on his own (he clearly couldn't against Switzerland), I'd like the second midfielder to help him out even if that makes him play deeper and also less threatening. A lot of people think differently - that Pogba being creative force in midfield makes him immune to criticism concerning his off the ball performance. I strongly disagree with that statement simply because playing an attacking midfielder in center areas doesn't make us a better team. I prefer team balance over individual brilliance.