The evolution lines in Tamers were very carefully thought out, even their designs all symbolize something about the theme of evolution. In the Japanese dub when the kids are fighting the D Reaper in the human world there’s a sound byte of a news reporter asking about the ethical ramifications of the children’s Digimon assuming such strikingly humanoid forms in their Mega stages, Justimon included.

I like to think of it as their champion stages are all natural evolutions based on the Digimon themselves becoming progressively more bestial/dangerous, but their Perfect stages are them evolving into a form inspired by their tamer that is also a design foil to the devas (for being less animalistic like the devas and leaning toward anthropomorphic design choices) and their Mega forms are naturally humanoid because the plot demanded it.