Since ESD faraday bags are conductive it's generally advised not to place your work on them. If you approach your work with an ESD charge on your body as soon as you touch the bag the charge in your body discharges to the bag which then conducts it to the work piece with potentially damaging effect.
But you don't have to touch it: when your body and clothing have a large charge it presents a strong electro-magnetic field. The bag acts as an antenna that, as your field passes over it, will pick up the charge from your body and conduct it to the board. It's not done any damage yet, it's just charged up the bag (and board) so that anything that touches it will discharge and do damage. So, in a way you could say it does attract ESD.
In practice consumer market motherboards are usually pretty well protected against ESD discharge. But that protection is mostly through connector pins, although many devices (like the CPU itself) will also have their own protection on their I/O pins. But a discharge presented to an interior interconnect (as where the bag is likely to be touching a board laying on it) could be more likely to do some damage.
I like to work on the bare table but cover it with a clean cotton towel to protect bottom side components and solder joints on bottom from scuffing. If I did a lot of this sort of thing a lot I'd buy an ESD safe work surface mat which isn't conductive but dissipative; it will slowly dissipate a charge rather than conduct it instantaneously. It's also safe to power up a device on the mat while powering it up on a conductive bag is a really, really bad idea.