Dropbox installer (Dropboxinstaller.dmg) fails to launch on Mac OS 12.3.1

Setting up a new MacBook Pro which has a clean install of Monterey. The Dropbox website transmitted a download of the installer (dropboxinstaller.dmg) to my Downloads folder. When I double click it, or right-click/control-click it to open the popup menu and select Open from the menu, the icon briefly looks like it is expanding with the genie effect, but the effect lasts for just a split second and then.... Nothing. No installer window, no objection for being signed or not signed, no information about what is or isn't happening. Just a stone-cold finder window showing the installer file, still right where it was, doing nothing. I am the admin with full permissions on this computer. Under security, about allowing installation of apps, I have selected to allow both app-store apps and apps with a valid signature. Nothing seems to help. It is so weird to just get a blip that sort of looks like a real response, but then nothing actually happens. No processes are seen as "not responding" to force-quit. Really obtuse!! If I go back to the website and select to use the desktop app, first I get a page saying "To get started" and then a big button "Launch Dropbox App" as if it was already installed. Clicking that button, I guess the site realizes it's not installed and says "To get started" and the big button says "Download Dropbox App". Clicking that sometimes does nothing and other times makes another redundant copy of the installer. Just like all the other copies, the installer does not open or launch like a normal .DMG should, no matter how cleverly I try to open it. I have tried modifying every security option short of disabling system integrity protection (god forbid that's what is needed, because really??) What's the deal? How can I install dropbox on this machine? And why won't Dropbox just put the app up in the App Store like so many other successful companies do? It would make it a lot easier, it allows for automatic updates, and (by and large) it actually does the installation without all this fuss.