I recently read the monster manual and every creature I came upon that had resistance or immunity to bludgeoning/slashing/piercing damage followed this with "from non-magical attacks" (may not be literal since my handbook is not in english)
What this boils down to:
What is the purpose of those resistances, since they are easily overcome by items experienced PCs tend to carry. Magic weapons.
Everything below provides context, but is not necessary to understand the question
The complete story behind this question is that I want to provide my players with a challenge in a dungeon. 5 monsters, and each can only be deafeated by the specific skills and abilities of one player.
While designing this I noticed this problem.
Since the point is NOT having the rogue sneak-attacking everything to death with his magic weapon, and I also prefer not to solve problems with houseruling I put this question.