@Stephen624 A host could have been a member since 2017, but used Airbnb as a guest for travel, and only just recently started hosting.
If there are reviews for the listing from 2017, and then none for 2018 or 2019, the host may have simply stopped hosting for one reason or another, and decided to start back up again. They may have had personal circumstances that made it difficult or impractical to host (ill family member, adult child or other family moving in, needed to do major renos). There's no telling. When people have questions like this, I just say, why not message the host and ask? Just say you're curious, it doesn't have to sound like you're suspicious of something strange. I've done the same with guests who had had accounts for a few years with no reviews. It turned out to be something totally straightforward, and they were lovely guests.
Depending on the location, it is possible that STRs are illegal where the host lives and he's asked guests to please not leave reviews so he doesn't get busted. And we're only 2 and a half months into 2019- there might be a longer term renter in there who booked from Jan-March and hasn't left a review yet. And not all guests leave reviews. So many possibilities. Just ask the host.