I couldn’t find any information as to why, but playing around with other symbols suggests it only does it with symbols where they assume the space isn’t supposed to be there. E.G. Colon, ending parenthesis, equals sign, etc. Digging around in the settings I couldn’t find any option to disable this functionality.
Folks elsewhere suggested switching to the Swype keyboard, but I don’t have personal experience with it in a very long time so I don’t know anything about the settings and automatic behavior.