Because this strategy is not sustainable in the Android ecosystem, not even for Samsung.
Google applies different levers here:
1. Google is making your feature a commodity: Samsung is aware that Google plans to implement a native version of the feature, with or without Samsung. Google will make the feature available to ALL competitors of Samsung. Samsung get's the chance to shape it WITH Google or will later be forced to ensure compatibility with it (because it's not sustainable for Samsung to coexist with AND compete against an ecosystem used by ALL other Android vendors)
2. Google offers to take over some tasks: Google creates media attention for OS-Upgrades, creating pressure for Device-Vendors to adopt the new OS-version as soon as possible. The more a vendor deviates from the generic implementation, the more time & resources (and money) will be needed to adopt a new OS-version. So it's in the interest of Samsung to contribute as much of their fundamentals back upstream, so Google themselves takes care of maintaining it.
3. Google may make it mandatory at some point: The Android Compatibility Definition Document (CDD) defines the mandatory requirements for a device to be granted into the Android Ecosystem. Google is in control of this document and may at some point add specific behavior or features as a condition for Android compliance. This "Desktop Mode" has the potential to become the default behavior for Tablets, so Samsung may be required to adopt it for devices classified as Tablet by the CDD [0]
4. Google returns the favor: Samsung can trade collaboration on this feature for business opportunities with Google in other areas (i.e. Mixed Reality, B2B, Chromebooks,...), which potentially allows Samsung to be first in an entirely new market...
Google applies different levers here:
1. Google is making your feature a commodity: Samsung is aware that Google plans to implement a native version of the feature, with or without Samsung. Google will make the feature available to ALL competitors of Samsung. Samsung get's the chance to shape it WITH Google or will later be forced to ensure compatibility with it (because it's not sustainable for Samsung to coexist with AND compete against an ecosystem used by ALL other Android vendors)
2. Google offers to take over some tasks: Google creates media attention for OS-Upgrades, creating pressure for Device-Vendors to adopt the new OS-version as soon as possible. The more a vendor deviates from the generic implementation, the more time & resources (and money) will be needed to adopt a new OS-version. So it's in the interest of Samsung to contribute as much of their fundamentals back upstream, so Google themselves takes care of maintaining it.
3. Google may make it mandatory at some point: The Android Compatibility Definition Document (CDD) defines the mandatory requirements for a device to be granted into the Android Ecosystem. Google is in control of this document and may at some point add specific behavior or features as a condition for Android compliance. This "Desktop Mode" has the potential to become the default behavior for Tablets, so Samsung may be required to adopt it for devices classified as Tablet by the CDD [0]
4. Google returns the favor: Samsung can trade collaboration on this feature for business opportunities with Google in other areas (i.e. Mixed Reality, B2B, Chromebooks,...), which potentially allows Samsung to be first in an entirely new market...
[0] https://source.android.com/docs/compatibility/16/android-16-...