Why does pip need an exclamation point to use in iPython?

This is actually not specific to pip, but really any shell command from the iPython notebook. You'll notice other shell commands also work (from the docs):

In[1]: !pwd
/User/home/

Change directory:

In[1]: !cd /var/etc

This is simply shorthand that the good folks at Jupyter have included. See Shell Assignment in the docs for more of an explanation.

When Python behaves as a system level command we use this exclamatory mark. Example: !python '/content/test.py'-> to run python file named test.py within a colab notebook