Why do devices need to be online for tracking in Find My?

If you use the Find My app on another Apple device (other than iCloud.com/Find), it may be able to locate your phone even if it is offline. See the following from Use Find My to locate your lost Apple device or AirTag - Apple Support

(and note in particular the last paragraph of the excerpt below)

If Find My shows "No location found" or "Offline"

• If Find My can't display the current location (because the phone is powered off or otherwise not sending its location), Find My displays the last location sent by the device or the last known location on the Find My network.
• If more than seven days have passed since your device last sent its location to Apple through the Find My network, Find My can't display a location and instead shows "No location found."
• If your device is online but is unable to send its location (for example, if there's an issue with Location Services), Find My can't display a location and instead shows "Not sharing location • Online."

The Find My network is an encrypted, anonymous network of hundreds of millions of Apple devices that can help find your stuff, even when it's offline. Nearby devices securely send the location of your missing device to iCloud, so that you can find it in Find My. It's all anonymous and encrypted to protect everyone's privacy.

You can use www.icloud.com/find to locate a device from a different Apple Account. See: Locate a device in Find Devices on iCloud.com - Apple Support

Note that the capabilities aren't quite the same when using iCloud.com/find as they are when using the Find My app. See the following from Use Find My to locate your lost Apple device or AirTag - Apple Support

• When you're using iCloud.com/find, you can't find your device's location if it's powered off, the battery has run out, or more than 24 hours has passed since its last location was sent to Apple.

Another option would be to setup Family Sharing and let it act like the other Apple Account is a family member's. You can then use the Find My app. See the following from that same support article:

If you're part of a Family Sharing group, you can use Find My on a family member's device. After you share your location with your family members, your devices are listed in their Find My app under Devices.

Learn how to find a family member's device