Are you sure Ax86 comes prerooted as 'stock'/ generic build afaik Ax86 trybto stay as close to aosp as possible, so prerooting wouldn't make much sense outside of test-builds.
In this case with RemixOS I'd say they chose to leave it esentially unrooted (with perhaps access to a su via a second binary), is primarily due to how they plan to run OTAs, but I guess we'll have to wait to see an OTA.zip