Now, that can be directly useful for e.g. mail or file servers on those platforms, but a lot of customers simply had to buy AV everywhere for compliance reasons. They'd be running the AV scan in cron checking for Windows viruses on AIX.
Discussions with those people were funny. They knew what they were buying was worthless and made that clear, we knew our product was worthless to them and made that clear. But their boss's boss had obligated them to buy AV, so sure, if they wanted it anyway we were going to make it and take their money.
I suspect Android AV exists for much the same reason.
- Android "administrators" (read: end users) are generally less likely to be cognizant of potential viral vectors than a traditional Linux/AIX/Solaris/etc. administrator.
- Android is capable of providing root access to applications should a user configure the device to do so, thus giving malicious software an avenue to break out of what's otherwise a pretty decent per-app sandbox. This is a concern due to the point above.
- Variation between Android installations (as far as malware authors might be concerned) is far less than the variation in Linux/Unix installations (especially server installations), so there's a higher probability of one exploit or vector being applicable elsewhere.
- Pirated software is more common on an Android device than on a Unix server.
A good AV solution on Android is therefore valuable, especially considering that Android malware does exist in significant quantities. It's less valuable if you've never rooted your phone and have never installed apps outside of the Play Store (or something else that uses its repository, like the Yalp Store), but still does offer some value beyond compliance reasons.