Cause it believes in freedom 🫶🏻🤣
Now, in all seriousness, humanoid basic characters are designed to serve as a base. The collection has lots of different animations available for the base characters, so you can find animations for all sorts of situations to be used in different game types. It wouldn't be possible to have all the available animations for all the "dressed" characters too (like the ones in the hero packs), because currently in the collection, there are over 90 animations available for the base humanoids (and counting), including animations for weapon attacks, spells, riding, crafting, gathering, etc.
Being naked is the price one has to pay for having such an expansive collection as Minifantasy 🤣
You can still edit the animations to make them fit with heroes or other dressed humanoids present in the collection tho