Why is my WiFi saying weak security?

Your iPhone is notifying you that the Wi-Fi Network that it is connected to does not have adequate security by today's standards. If you are interested in improving the security of your Wi-Fi then you can log into the Router and change the security settings to WPA2 (AES) in the least. If you do not know how to log into your Wi-Fi Router then please refer to the owner's manual. If you do not have the owner's manual the please contact the support team of the Router's manufacturer / provider.

More information here ➔ Recommended Settings for Wi-Fi Routers & Access Points. 

Axel F.

Because you can "do better", using a password type more secure (depending on your modem-router settings).

Contact your Internet Provider and/or your modem-router manufacturer support team and ask them.

Regards

Giulio