Plus as some people already mentioned, tablets from couple of years ago still run quite well right now. Especially newer iPads.
For me it's TV. While I own a TV, I essentially never use it. A tablet has more viewing area than a phone, which makes it much more pleasant to consume TV shows on.
I think you hit the nail on the head re. how well relatively recent tablets work. At first I thought a tablet would be like a phone — replaced annually — but I find that I've had the same tablet for four years. It does its job, it's a nice size (and Google haven't come out with another 7" tablet since!); why replace it?
I rather prefer 4.8 inch smartphone and a 10 inch tablet. A tablet is much nicer to consume websites, type in short text and watch videos - also less eye strain.