Why is this called ricotta?

Technically you are correct. True ricotta is traditionally made using the whey left behind in the cheese making process. It's also not actually considered a true cheese, because it is acid coagulated versus rennet coagulated- and you are capturing a different sort of protein. But most folks don't have access to the gallons of whey you would need to achieve any significantly usable amount of ricotta. Ricotta is also drained of excess liquid, even in the traditional methods. I assume even in the "old days" it didn't take some Italian grandma long to figure out that she could add additional milk to the reheated whey to get a bit more of the product. What she did not use was citric acid or lemon juice, relying only on the acidity of the whey to coagulate it. However, since most of us don't have gallons of whey leftover from making our pecorino or parmesan- we must make do with what we do have. So it's not "genuine" ricotta, and would be related to paneer. It isn't a cottage cheese- that uses a culture and rennet to produce. Paneer requires pressing the curds, which isn't done when you want the ricotta like product. You could however, let it nearly drain completely without pressing, salt it and age it a bit to obtain ricotta salata. What this recipe gets you may not be strictly traditional ricotta, but it's still miles above what you would buy in the shops. Even the Italians don't seem to have a problem with it done this way- and while they will fight you over where your Parmesan originated, they don't fuss about ricotta. While there are a very few still making it the old way, most add in additional milk, and also use an additional acidic coagulant. If you prefer to call what you make a ricotta-like cheese, that's fine. It's the taste that counts, not what you call it.

It seems to be one of those changes that happen. Buttermilk started out as the liquid left over from making butter, but I am pretty sure the stuff being sold in stores never saw a churn. Looking at recipes for Ricotta, they all seem to start with milk (and cream sometimes) rather than a gallon or two of whey. The whey is used to make nutritional supplements and animal feed.