As a developer it is currently tedious to use attributes when storing data related to things that use spaces in their names.
For example, my game has a plethora of swords, many of which have spaces in their names. I like to store data about them in attributes under the player, describing how many of each sword a player has collected as well as other info.
This is only one example. My game uses attributes heavily and the inability to use spaces in attribute names is constantly bothersome. Gamepass and product names, pet names, quest names, names for different areas within the game, all often have spaces in their names. The method I currently use to get around it is to convert all spaces into _ characters, but this is both performance intensive and tedious.
Considering attributes are entirely separate from properties, and can only be accessed through a function with a string, I see no reason for spaces not to be allowed.