Honestly GG, I thought the same before I purchased it because they don't necessarily do new versions yearly, or even every other year, as other software companies do. I had tried out the 30-day trial back in 2014, but didn't want to spend $159 for it, and I hadn't seen a less expensive sale at Nevercenter than $109.
That said, Steam had it for about $69 at the time, but I don't do Steam or it's games, and at the time their version was one version behind what Nevercenter was offering, so I just waited. Then Netherworks had posted on the old RDNA forums that MacHeist had it for a lot less (I'd have to check to see exactly what it was), and it was offering a bundle of Mac apps with it, which I didn't need as I'm on a PC, but all I did was purchase the license from them, ignored the download of the Mac version of Silo, and went to Nevercenter and downloaded the latest PC version. The new license I had purchased worked perfectly on the 30-day trial version I had originally installed, and then I upgraded to the latest version I had just downloaded.
So, they may not update it as regularly as other software, but there have been updates, and now we have the 64-bit version, which is always a plus.