That’s because conda’s base environment is activated on startup.
If set the auto_activate_base parameter to false, it will instead default to the system environment and avoid the prompt. To do so type:
conda config --set auto_activate_base false
Edited 2021/09/09:
If you are facing the exact same situation as the OP, that you are using conda to manage environments, and wanted to make (base) environment looks no different from the system environment in terminal, check @merv ‘s answer for the procedures. Note that the prompt string is stored in a certain special variable, depending on the shell you are using, so check the documentation of your shell if it does not work for you.
If you want to use the system environment without conda as the default, my original answer was the solution for you.
Thanks to @merv and @Neinstein for pointing out in the comments.