What are the reasons for Flask's warning about the inadmissibility of using the built-in server in a production environment?

Flask warns against using the built-in server in a production environment for the following reasons:

• The Flask built-in server is intended only for development and debugging. It is not optimized for high load and multi-threaded operation.
• It lacks support for scaling and load balancing.
• The built-in server does not provide a sufficient level of security for public applications.
• It may be less resilient to errors and attacks.

Therefore, in production, it is recommended to use full-fledged WSGI servers such as Gunicorn, uWSGI, or others, which provide performance, security, and stability.