Because past Zone 1, it seems like 50/50 that a sword upgrade even exists outside of the merchant (often verifiable with the wand map), and another 50/50 that it's even remotely reachable.

EDIT: It is, and seems to be built around the expectation of buying at least the sword and shield every time. Anyone going after the maze achievements, do a Genie run of Story Mode and save right before fighting him. You can repeatedly reload and kill the genie for 10 soulstones apiece.

Furthermore, the bosses seem to spawn 50/50 either with a red door and normal stats, or no red door and double stats. The problem is the maze isn't actually tuned around a boss with blindly doubled stats. In earlier zones it's usually easier than hunting down a red key, but by Zone 4 the double-stat boss is nearly impossible, and in Zone 5 you straight up fall about 80 points short of being able to damage the double-stat boss at all, nor does it have an altar to go past the gem drops. It was really very poorly implemented.