you can't simply classify servers or employees categorically as direct or indirect costs, but must consider how the inputs contribute to outputs. a janitor is a direct cost for a cleaning business but not for most others. if you're aws or ibm, then yes, (fractions of) servers are a direct cost. but servers are too course of an input, and non-linearly related, in most saas businesses (e.g., stripe or hubspot) to be apportioned to individual customers. that's not the same as being able to statically divide server costs by the number of customers to reach a per-customer cost, which you can do with any business and any cost.

take an accounting or pricing class if you're really interested in this stuff. that will explain it much better and more in depth than a few sentences here can.