You guys are all ignoring the true reason. While it's true that 'human' is a convenient form, it is hardly the most ideal. What is the most ideal form? I am not 100% certain, but it is not human. Why do monsters not transform into slimes or tentacle monsters instead? The limbs can be used to pick up and control objects just like a human? Their bodies have even more limbs. Potentially stronger/invulnerable to attacks. Is this not the ideal form?
Or why not a human with four arms and four legs? Twice as many swords and twice the balance!
Therefore, the real reason we see 'human' beasts is simply because they're easier to relate too. It's much harder to emphasize or imagine yourself as a dragon than to imagine yourself as a human that can turn into a dragon (even if it's really just a dragon that can turn human). It's all a device to tug at the readers feelings for the characters. It's probably easier to write too. Much harder to explain the adventures of immortal-kun and his friend dragon-kun if dragon-kun doesn't fit inside literally any building ever. Lol.
But, for the sake of argument, let's manufacture a reason. In almost every webnovel I've finished, the 'final immortal' is human. When MC-kun breaks through that final barrier, ascending to the highest possible dimension with almost nobody else even existing on that level yet (if any at all), the other characters tend to be human or human-shaped. This implies that the 'creator' of MC-kun's universe is biased towards humans. It's only natural that the creator views human as the ideal form. In his magnificent generosity, he bestows that form on the lowly beasts that rise to a high enough level.
Now here's an idea. This would make for a good novel. In an alternative universe created by a beast 'creator', the dragon is the ideal form. In this universe, humans are viewed as lowly uncivilized beasts. However, humans and other monsters that reach a certain level will be bestowed with the grace of the 'creator' and be gifted a dragon form. Now, when human-kun enters a palace, he is no longer as tiny as an ant. He is a big dragon and everything is suited to his current size, limbs, body, etc. Thus, the answer to the question is the bias of the 'creator'.
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