It has to do with a plug-in that only works for IE.
You can try to override the "not secure" thing and see if it lets you.
Regarding the plug-in, you have to remember that we are not the Dahua and Hikvision intended audience - it is the Dahua authorized installers hoping you will subscribe to their monthly fees to "manage" the system.
While newer devices are supposed to work with other browsers and not need the plug-in nonsense, keep in mind to ensure full compatibility, IE is still preferred. And this isn't specific to just Dahua or Hik either. And sometimes the plug-in nonsense is needed.
It comes down to IE was the most popular browser when these cameras started to be made, so they centered the firmware around one particular browser and they got lazy and never updated the internals of the program to play nice with other browsers as more became available and IE started to fade.
Back when the firmware was written, it was probably a pain to get it to play nice with every different browser, so they went with the most popular one.
They haven't had a need to address this because their intended market (remember it isn't us) is mainly businesses where they have enough light they can stay in default settings so they don't have a need to login to the camera via browser. It is us homeowners that push these to the limits and actually change settings.
As long as their intended market simply uses the NVR as a recording device and to display it on a monitor, they have no need to change what works for their intended market.
Further, Steve1225, who is an installer and works closely with Dahua, said it best here in this
post:
Even though Microsoft has sunset Explorer, it is still available in Windows 11 (hidden). Many older programs rely on the old Trident (HTML-) Engine from IE, even stuff as basic as the old Control Panel. Old browsers are old, so unlike modern browsers where the WebView can be its own component, it's either all or nothing. And since the engine is still required not to break Windows functionality and apps from before 2015, it's still included (but mostly hidden). The whole file manager system is still based on IE. That's also why Device Manager and Control Panel are still there too. Similar to Dahua and Hikvision, Microsoft changed the UI and not the core, and added new features to upgrade its OS. A lot of legacy stuff are hidden.
So contrary to popular belief, Internet Explorer isn't going away anytime soon.