Recently there has been a trend to move to more powerful calibers (5.56 NATO standard round being out gunned and out ranged by 100 year old Enfield .303) so even Level III protection is getting past its sell by date. i.e. .338 Magnum Laupa will zip through 30+lb ceramic plated vests @1000 metres.

Trading immobility for dubious protection has to be a very specific scenario question. Good camouflage is very lightweight by comparison. Not being around when the shooting starts is even better, hence the SAS moto, 'Run away, run away, live to fight another day' 30lbs of ceramic plated vest isn't conducive to running away very quickly.