Maybe it's partly because of my age that I enjoy listening to R-18 content more, but that doesn't mean I don't enjoy all-age and R-15 content. It's probably because I feel like the seiyus for all-age and R-15 titles are people I recognize from anime or games, so I feel like I've heard enough of them from those sources. Unlike R-18 titles, which mostly use pseudonyms and I don't even know their real names, which is what makes me intrigued with their works.
Actually, I've noticed that there aren't many titles released in the all-ages category, but looking at Animal Planet as an example (which, as you may know, operates under the name Lamina Planet in the R18 section), they're still able to release all-ages titles. But this is just my thought—maybe seiyuus who can do all-ages work might be busy with other projects like anime or games, or maybe the creator itself thought had enough of otome games combined with drama CDs tokutens.