These two statements seem contradictory to me. "Healing is fairly normal, but too expensive for most people." - that means it's not normal for most people. For most people healing potions are like an emergency room - yes it exists, but not something everyone has easily available. PC's are the rich guys with the premium health insurance (i.e. a cleric who doesn't charge for healing).
In most of the published adventures and hardcovers - the monsters and bad guys don't have healing spells and potions in their stat block. A few do, but most do not.
Under the assumption that they are not fighting PC Adventurer's 100% of the time, but rather mostly fighting other creatures of similar ability - they usually wouldn't have to deal with opponents who can easily heal during a fight.
And most of the creatures you fight are not "modern warfare special forces".
PC's are an exception, assuming everyone else has the same options and abilities leads to a skewed view of the world.
For home campaigns, adjust this to whatever makes sense for you (I certainly do). I'm just saying that this seems to be the default assumption in the rules and published adventures.