At risk of belaboring an already often discussed and frequently debated aspect of the wizarding world, why is the wizarding world so comfortable with elf slavery?

This does not make sense for several, I would hope are rather obvious, reasons:

1. The Wizarding world should be readily aware of the history of slavery in the "muggle" world. Some witches and wizards of mixed muggle parentage themselves would perhaps have ancestral knowledge of things like American and European slavery in their their own bloodlines. This immediately seems like a tremendous cognitive dissonance that they would on one hand be readily aware of this aspect of human history and at the same time perpetuate basically the same thing themselves.
2. Wizards themselves remember their own adverse history of persecution with muggles. This seems to call in to question why they would systematically oppress another sentient magical race. Whatever they believe their positive benefits are of this system.
3. If House Elves do not desire or even enjoy freedom or in fact (as many magical folk claim) enjoy their servitude, why do they rejoice as Dobby did when treated kindly or given freedom? This seems to imply they would prefer not to live in bondage and they are treated as inferior creatures to magical folk simply by virtue of the fact that they are treated as subservient. This despite all evidence we see that House Elves being intelligent, sentient, capable of magic no less powerful than wizards or witches and do indeed desire equal treatment, kindness and freedom.