Advancements in platter density have largely stalled for the better part of the last decade. Your 3TB drives likely used three 1TB platters, and today's do as well, so they cost roughly the same to manufacture as they did then. Many of the higher density drives are using somewhat higher density platters, but they still mostly get those higher densities by adding more platters, thus increasing the cost to manufacture them. 8TB drives generally use five to six platters, roughly double what's in your current drives, so they will naturally cost more.
Be aware that some recent drives with higher-density platters will also use shingled magnetic recording (SMR) to get those capacities using fewer platters, and SMR has a negative impact on performance. It has the potential to significantly cripple performance when performing random writes to the drives, as they overlap data in such a way that they need to overwrite a relatively large chunk of data each time a small amount is written. If you are writing large files once to a drive and only reading them afterwards, like for video storage, then its probably not a big concern, but for tasks involving lots of small writes and rewrites, then you may want a tradition CMR drive, which will often cost a little more due to them using more platters for a given density. Some product listings have actually started noting whether a drive is SMR or CMR now, since the manufacturers recently got called out for quietly swapping in SMR platters in place of CMR ones to cut costs, while keeping the existing model numbers. Of course, if you are shucking an external drive, then it will be harder to tell exactly what sort of drive you will be getting inside.