The short answer is that Adobe transitioned to a subscription-only model years ago, meaning the software is only available via recurring monthly or annual payments. The Disappearance of Perpetual Licenses
Adobe discontinued perpetual "buy once, own forever" licenses for its professional software line on January 9, 2017. Prior to this, users could purchase as a standalone product for roughly $700. Today, the "latest" version of InDesign is tied exclusively to the Creative Cloud (CC) ecosystem. Current Official Purchase Options buy indesign outright
Because a one-time purchase is no longer an option, you must choose from one of the following subscription plans on the official Adobe InDesign Plans page: The short answer is that Adobe transitioned to