I believe they were a mission church without headship, as were many of the new churches. So as in all the epistles Paul used them to teach doctrine to the babes in Christ in Rome and then that letter circulated through all the churches and to us today. Yes it is still teaching sound doctrine to babes in Christ today :)
Then Paul did not recognize a church in Rome in that letter
That would explain why Paul does not address "To the Church at Rome" like say: To the Church at Corinth, or the Church at Thessalonica, or the Church at Ephesus