The XRPL Labs team carefully decided not to ship a desktop client.
Desktop environments are harder to keep secure, more easily compromised (keyloggers, viruses) and 'less personal' than a phone. Desktops can accidentally stay unlocked while phones are carried on users.
On top of that, XUMM implements a flow where one can use any client securely, while signing happens on the mobile device, making it possible today, without a dedicated XUMM desktop client, to use XUMM for signing & desktop Web-apps for XRP Ledger interaction.
You can use those clients on your desktop, while signing in and signing with XUMM using a QR code or push notification. Best of both worlds: the security of your smartphone & the convenience on your desktop.
Some of the clients supporting XUMM for sign in and signing: