Why Can’t You Mutate Python Strings?

Performance: knowing a string is immutable makes it straightforward to lay it out at construction time—fixed and unchanging storage requirements. One reason tuples and lists are different is this. The implementation can safely reuse string objects. In CPython, single-character strings are pre-allocated and string operations that don’t affect the content return the original string.
Python treats strings like numbers. No activity will change 8 or “eight” in Python.
In this blog, we will cover all the details about the immutability of strings in python.

Source: https://insideaiml.com/blog/Python---String-Immutability-909