pip is a module in the standard library, but it isn't a built-in function(or module), thus you need import it.
pip is run from the cmd line, not from Python interpreter. It is a cmd that installs modules so you can use them in Python. Once you have installed a module, then you can open the Python shell use it.
The Python shell is not a command line, it is an interactive interpreter. You type Python code into it, not the commands.
Python 3.6.5 (v3.6.5:f59c0932b4, Mar 28 2018, 16:07:46) [MSC v.1900 32 bit (Intel)] on win32
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pip
Traceback (most recent call last):
File " pyshell#0", line 1, in module
pip
NameError: name 'pip' is not defined
import pip
pip.main(['install','requests'])
Why does “pip install” inside Python raise a SyntaxError?