Why does the Android app not support TAP-style tunnels?

The Android VPN API supports only TUN-style or routed tunnels on Layer 3 at the moment. TAP-style or bridged tunnels on Layer 2 are not possible on Android. This is a limitation of the Android platform. If you try to connect a profile that uses a TAP-based tunnel, you will get an error that says only Layer 3 tunnels are currently supported.

If you really want to see TAP-style tunnels supported in OpenVPN Connect, we would encourage you to contact the Google Android team and ask that the VpnService API be extended to allow this. Without such changes to the VpnService API, it is not possible for non-root apps such as OpenVPN Connect to support TAP-style tunnels. Currently we have very little demand for this feature because Layer 3 is for a number of reasons the better choice anyways.