You're right, they don't because radio has stopped playing those early hits. Because of this, many have resorted to other media for their music. Radio isn't it for them, which is unfortunate.
If it were available on radio, many would listen.
55-64's listen to radio about as much as the two younger cells, 45-54 and 35-44.
The fact is, as I have said before, is that not all people in 55-64 listened to Top 40 when young. This may be because of ethnicity or because of other musical influences such as country and other genres. So those people are very unlikely to listen to "oldies" as they did not grow up on that kind of music.
And then there are many fans of Top 40 in the 60's who have moved on from hearing the same songs they grew up on and have migrated to rock formats, AC formats, country and a variety of other options. And many listen to less music and more news and talk.
The listening in 55-64 is quite healthy. But since the appeal today of 60's oldies is nowhere near universal (and it never was), not having oldies stations for that music really does not seem to affect the usage of radio by those seniors in a significant manner.