He's instructed to do it because Mourinho isn't an idiot that would try to turn Smalling into a good passer.
When Rojo, Lindeloff and Bailly play they regularly try to find the forwards with their passing.
You comment about Xavi and Iniesta is clear ignorance as Mourinho has built the play with centre backs whenever he has them available. He is not someone that is so dogmatic to try to force passing from the back or vice versa. He adapts to the personnel available
The Xavi/Iniesta part was obviously hyperbole.
But I disagree about the part about Mourinho adapting to the personnel available. Mourinho has his system of play, think of it what you want. It can be really effective when it does work and it can be a complete bore when it doesnt work. He has always played basically the same way with all his teams though IMO. I dont get this narrative that Mourinho historically has been an offensive coach or that this (offensive thinking) is some latent trait of his that is just hindered by these crap footballers at his disposal at the current United. IMO we play the way we do because its the way Mourinho knows how to play and also thinks is the best way to be successful.
I dont know if I am overinterpreting your post but you seem to be one of these people that believe that United play the way we do not because of Mourinho, but in spite of him. And that he actually dreams of electric sheep, sorry: offensive football at night....
I will not go in in detail on what I think constitutes the basics of Mourinhos system more than to say (since this started with Smalling) that of the axioms is to have the CMs be responsible for circulating possession, and most importantly: have the CMs "below" the first opposition pressing line. And no more than one FB in front of the ball. It effectively means that a Mourinho team always willl have five or even six players behind the ball if possession is lost. This is fundamental for Mourinhos way of playing and why his teams is so good defensively.
On the other hand: it means that the options for playing the ball forward are limited and why a lot of times our circulation of possession ends with a long ball to Lukaku.
I will state a couple of arguments in support of my opinion that the way United play now is very much intentional and has nothing or very little to do with the personnel available:
1. Mourinho has always set up his teams like this. For me this applies even to his great Real Madrid team in his second season there. That was a great example of how a Mourinho team can function when he gets his players onboard. Offensive football it was not though even if they scored a lot of goals.
2. How he has set up United to play under his first two years here; I really dont need to go into this in more detail than to state that the best argument for this being the desired system of play is hanging on to it for two years with no sign of changing it.
3. Player recruitment and retention: Look: if Mourinho wanted to make us play a different type of football, he would have bought completely different players than he has. If the idea was to play with a high line and a high press for example, both acquisitions of Zlatan and Lukaku would be completely off. Great players as they are in Mourinhos system they dont have the mobility or stamina to play a high pressing game. See Zlatans exit from Barcelona. Same goes for Matic. Great player in a defensive system, dont think he would get a sniff under Guardiola or Klopp.
Then there of course is the reliance on Smalling. And the new contract to Fellaini. Etc.
So my opinion is that United play the way we do very much because its how Mourinho wants us to play. Not because he is forced to by the available player