Maybe if the letters weren't specific in who they were written to and containing information that was relevant primarily to churches in those cities.
Take the letter to Pergamum for example. First, it is specifically addressed to the people at the church in Pergamum:
2:12 “To the angel of the church in Pergamum
If a reference to a specific location isn't enough, then secondly, he refers to a specific individual, by name, who was martyred. Third, he refers to the city of Pergamum as a "city":
2:13 ‘I know where you live – where Satan’s throne is. Yetyou continue to clingto my name and you have not denied yourfaith in me, even in the days of Antipas, my faithful witness,who was killed in your city
Fourth, he refers to specific issues the church in Pergamum was facing, including dealing with the "teachings of Balaam" (the same teachings about eating food sacrificed to idols and sexual immorality that Paul and the Jerusalem Council also dealt with) and the teachings of the Nicolaitans (who existed in the first century church):
2:14 But I have a few things against you: You have some people there who follow the teaching of Balaam,who instructed Balak to put a stumbling blockbefore the peopleof Israel so they would eat food sacrificed to idols and commit sexual immorality.
2:15 In the same way, there are also some among you who follow the teaching of the Nicolaitans.
You can do the same type of analysis with the other churches as well. It's fairly easy to see they were written to, for, and about 7 churches physically located in Asia Minor in the 1st century. This is just simple literary analysis identifying author and audience - it's not hard.