Oracle linked servers require installing the Oracle provider software on the SQL Server that will have the linked server. It's not trivial, but it's not that bad.
Then security has to be set up for the Oracle login that will be used, firewall ports, etc.
At my company for instance, we pull SAP data from Oracle into SQL Server so it can be analyzed with various reporting tools. If you have developers who are skilled in SSAS, SSRS, PowerBI, QLIK, Tableau, etc, they probably want to get the data out of Oracle and into SQL so they can play with it. Not to mention the endless spreadsheets. Theoretically, all those consumers could connect to Oracle directly but then your installing and maintaining Oracle drivers all over the place. Office products and just about everything else can connect to SQL Server with no problem.
The Oracle team will have to set up security for whatever data the linked server is supposed to have access to. On the SQL Server side, a linked server can have security settings so that only certain logins will be able to use it. But a linked server open to all users would expand the Oracle security surface, possibly more than they would like. In some cases an application connection may be better from a security analysis perspective. If the data is sensitive, and the Oracle team and/or data owners control the application, they have a better idea of how the data is used than if they allow linked server access.