Because the fleet is not finished, and a one-time price for an unfinished thing forces a dishonest choice: either freeze the product at the version you bought, or charge you again later and call it an upgrade.
Agents are still being added. Existing ones change as real audits push back on them. The knowledge graph that will sit under all of them is being built now and is included in the fleet price as it rolls out, at no change in price. A subscription is what that arrangement actually is.
The single agent stays a one-time purchase for the opposite reason: it exists so you can find out what this way of working is like without committing to the system. A trial that expires would be a worse version of that, so it is a real purchase with a real limitation, stated plainly. You keep the version you bought, and later releases are not included.