People don't use single browser and single email client, why IM should be different? It's a deficiency when you're forced to use a single "official client".
But most people seem to use Chrome on every device, no matter how much I wish they didn't.
Google has since spent the time making Chrome more locked down and user-hostile from a privacy perspective but the conventional wisdom for years was “just use Chrome” (for good reason) and that inertia continues :/
Also last I checked - on iOS devices even if you use Chrome, you're really using Safari - I believe apple forces all web content to be rendered using the built-in WebKit.
You sign up for Gmail, you get the Gmail app. You sign up for Outlook, you get the Outlook app. 1 service = 1 app.
For browser - on mobile most people use whatever the default browser on their device is. On desktop - Chrome. Pretty much everybody just downloads and uses Chrome.