Straped on vs Sewn in armor
No doubt there are some disadvantages with sewn in armor and you pointed out one of them.
But think of the realities of long multi day and multi week rallies in that the elastic will constantly rub the skin area to a point of causing irritation. Yes - you can use powders to reduce the friction between the elastic and skin but eventually long hot sweaty multi day riding will cause friction which creates irritation.
All of my pants have sewn in with exception of my MX pants