You wouldn't want to. That could and would result in nightmarishly bad mistakes and directions. You'll never see route drawing in iOS unless it is "Genius Routes" or something like that, where it takes your drawn input and calculates a safe/reasonable approximation of what you were trying to do.
But I don't see much advantage anyway. When I map directions to a location and I know one or two roads I'm definitely taking, but they don't show up as part of the route, I just go my way and it automatically recalculates based on that decision.
Can't see much other efficient use of route drawing.
In some cases, I sure would want to! ie when I travel from the northeast down south, I don't want to go through NJ and Dover MD and that's where it puts me. Quicker time-wise and less grey hair to go a little further west than straight down. I would like the ability to manually map that change. If I just start driving, for all I know once I get through PA it will try to shoot me back over to NJ or something. Maps can be pretty stubborn about its route and it's kind of hard to see what it's thinking while I'm driving.
If I'm running around town or within a few towns, I would just pick my route and let it complain. Not on a longer trip, now that's risky. Maps sure can't tell what is a safe area or the fact that you may sit at a toll booth for an hour if you don't have an express pass. Safe is relative to the person doing the traveling too. I don't like cities and prefer backroads.
That's why I like to do my own tweaking. But I was brought up with AAA Triptiks so I understand how to use a map