The cost of operations doesn’t just include server hardware (or renting of cloud VMs) or internet connections and cost of egress traffic. It also includes the time these SPOs are putting into the support and operations which is severely underpaid at the moment.
Also, keep in mind, these margins can be changed by SPOs at any time. Good chance this will happen for most, if not all 0% pools. The ones I’ve seen only have 0% for an introductory period.
With that said, let’s assume the SPO can maintain 0%… the cost to delegators from 0% to let’s say 3% isn’t so significant as to make the 0% more intriguing vs the risk that it will someday disappear on you. Finally, you may chose to support a pool with some ADA that let’s say is at 6% (just throwing numbers for the sake of the argument) where the pool is using profits for a good cause for example, or running on clean(er) energy or … take pick your… obviously there’s the verification and vetting that this is where the profits above and beyond operation costs are going to etc…
In short, not as simple as a straight up margin calculation; there are quite a few factors and uptime/sustainability is in my mind at the top of the list.