This was in the Onedrive web client.

Assuming the worst of Microsoft (which I think is reasonable), here's my theory: They want you to install the app rather than use the web client, that way they have more access to your phone or computer. This was also seen in Firefox and I haven't bothered trying it on Chromium, so it could potentially also be similar to the "anti-Firefox" tactics that Google likes to use (maybe Microsoft too since their new Edge browser uses Chromium).