But why? Why is a goblin always weak, but an elf could be anything? Why can't a goblin be powerful? Exactly how do you decide the power of each creature? You can't use the rules, so, what are you using?Pretty much "yes" to all of those, unless there's something very odd going on.
Again, this is a common way for new players to get into trouble in my game. They would runs into the room and ignore the goblins. Oh, what, each goblin is a 5th level ice barbarian...well they players "never thought" that could "ever happen".While there's nothing to say one of the Goblins isn't a trained 14th-level Fighter, given that the PCs are meeting a Frost Giant the odds are extremely high they've already met and dealt with numerous Goblins in their previous adventures; and odds are just as high that 99+% of those Goblins were rather pathetic as opponents except when the PCs were still 1st level. So, Goblin = mook in the eyes of both characters and players, until and unless one shows it's somehow got more going for it.