Why does WiFi say weak security?

Usually because your Wi-Fi security is weak.

Recommended settings for Wi-Fi routers and access points - Apple Support

This usually involves re-configuring the Wi-Fi router, maybe updating Wi-Fi router firmware if the vendor has an update available, or replacing what is a very old and limited and slow Wi-Fi router.

Or ignoring this message entirely, as the only thing that has changed here is your awareness that that your Wi-Fi security is weak. Your Wi-Fi will continue to work as it has.

Right below Weak Security it explains why your router’s security settings are not secure.

This is NOT an iPhone problem; it is a problem with the settings on your Wi-Fi router.

A "Weak Security" notification means that your Wi-Fi router is not configured correctly. There is a new feature in iOS 14 and later and Mac OS Big Sur (11) and later that checks and displays a message if your router is not using the most up-to-date security settings. Note that your router has always had weak security, you just didn’t know about it until now. 

See this Apple support article for how to configure it correctly—>Recommended settings for Wi-Fi routers and access points - Apple Support, specifically the section headed Security. And don't forget to configure your range extenders (if any) also!

After you change the router settings you should Forget the network in your phone settings and reconnect.

Note also that weak security does not interfere with using Wi-Fi, it is just a warning. Your Wi-Fi should still work normally, and, if it doesn’t, it is not because of the weak security message.