You are not (I don't think). Back in the day (way back in the 70's and 80's when dinosaurs battled hairbands across the interstates) when D&D was young, most of the XP was gained for Treasure. Monsters are scary! I mean in more recent (3/4e) editions encounters are balanced so a PC can with fair confidence charge whatever it is they need to fight.
But this is insanity. I mean seriously, if you see a 14 foot monster with a club bigger than you that uproots trees which idiot runs straight at him confident he'll survive? If you were a real person in that story, you'd run away (as they do even in Lord of the Rings until they're cornered in Moria). So DW lets you do fictional positioning. I mean hacking and slashing a giant can bring him down, but it might be better to defy the danger of him smashing your puny-bug butt and lead him to a cliff where he falls off, using only 1-2 rolls with some assists from well placed arrows, or friends causing distractions.
This may be a 'gear' switch in the mind. What are your goals? If they are to systematically kill everything in the dungeon? Then patience, careful waiting for groups to become separated, several volleys from the dark before engaging certainly seems more logical than 'CHAAAAARGE!'. If it's to get the secret treasure, you don't have to fight every time. If you do choose to, well then you're brave and epic (and maybe just a smidge crazy) because you stare death in the teeth every time. And if you die bards will sing about you (probably).
I personally don't feel this is something you're doing wrong. I also don't think it's anything your players are doing. Fighting people with swords who want to kill you shouldn't be a guaranteed success always. I mean, a shiv to the belly in a barfight will kill even the stoutest person. Battle-hardened or no. Taking away the teeth of combat makes it less meaningful, and encourages less diverse thinking.