well creating a first person shooter that is also a third person shooter is not easy task. a lot of resources needs to be put in there and a lot of new animations and mechanics tweaks.
and obviously there is no manual leaning, for reasons i already stated (too problematic with controllers and developing a PC only feature is a waste of money in the developers eyes).
In Fallout 4 there was a mod that allowed to lean from farther away from cover. i wish UBI's games were mod-able (FC3 and 4 were but there werent online coop) so maybe someone could increase the distance form which you are "considered in cover and can lean" becase it really does feel like you need to glue yourself to a wall pretty tight, which isnt necessary, gameplay wise.