Because according to the FAQ, /session isn't suitable for my use:

• You will definitely want to use qemu:///system if your VMs are acting as servers. VM autostart on host boot only works for 'system' [Yes, my VMs are acting as servers]
• the root libvirtd instance has necessary permissions to use proper networkings via bridges or virtual networks. [Yes, I use OVS, with quite a complex bridge+VLAN system configured at boot]
• qemu:///session has a serious drawback: [...] the only out of the box network option is qemu's usermode networking, which has nonobvious limitations, so its usage is discouraged.

(Source: https://wiki.libvirt.org/page/FAQ#What_is_the_difference_between_qemu:.2F.2F.2Fsystem_and_qemu:.2F.2F.2Fsession.3F_Which_one_should_I_use.3F)

So I have to use /system, according to the FAQ.  But it'd be nice to nail the daemon down to reduce the attack surface.

- Peter