> you sell a product where it's easy to determine the cost for one user signing up by themselves, so you figure out the required markup and publish that on your site.

Then someone at a large organization can multiply this number by the expected number of licenses they'll need, and get a ballpark estimate for the (upper bound of the) costs of the service, which is a critical input in determining whether it's even worthwhile to consider talking to the vendor. Having that information, the organization can then schedule a call to negotiate whatever extra adaptations and discounts they need, or realize signing up is unlikely to have positive ROI and skip it, which also saves the seller from wasting their time on a deal that won't come through.

Vendors that hide critical information and pricing behind a phone call are eating the risk of having their time wasted on negotiating deals that would never succeed, trading it for a chance to scam some clueless or loss-insensitive companies for some big money.