> For example, JetBrains ships its own Java runtime for all of its IDEs.
That's actually the recommended way for Java applications nowadays. There is no official current JRE for end-users to download anymore - the newest version you can download from Oracle is 8 update 281. JDK is currently at version 16.
I don't know the exact thought process that led to this decision but I know that people were incredibly annoyed by the "you need to update Java!" notifications and that applications were rolling their own JRE for quite some time to get around outdated JREs on customer's machines.
That's actually the recommended way for Java applications nowadays. There is no official current JRE for end-users to download anymore - the newest version you can download from Oracle is 8 update 281. JDK is currently at version 16.