Throughout the history of Tamriel, the Orc capital of Orsinium has been sacked countless times, and I read somewhere that it was sacked sometime in the 4th Era. Why do the Bretons and Redguards hate the Orcs so much? My interpretation of it at least is the Bretons and Reguards use Orsinium as a scape goat whenever things don't go their way.

Because they're Orcs. That's the point.

At the same time, the Bretons and Redguards have been doing this for thousands of years and you would think that by now, they would know that Orcs aren't some mindless beasts that need population control.

They're ugly green men who run about chopping poor adventurers up and stealing children from cradles.

Who wouldn't want to kill 'em?

That's the humans view on it. From what I have read and heard, the Orcs living in Orsinium are pretty civilized.

Yes, that's the human view, and that's why the humans kill 'em.

Which is [censored]. I mean how many times are they going to sack the damn place? To me it's pointless. What do they get out of killing all of those Orcs?

It's all due to history.

The Orsimer, for the longest time, were considered on the same level as goblins and spriggans. Monsters to be culled. Orsinium, at that time well within High Rock, was basically the largest nest of the monsters. So, raids to Orsinium was basically an atempt to purge Tamriel of these warlik barbarians.

Then came the Dragon Break of 3E 417. Among the many changes, Orsinium became recognized as a member of the Empire, with all rights due to its people (the Orsimer) as are guarnteed to all citizens. This is a direct result of Lord Gortworg's petitions to Uriel Septium VII, and the reality-warping nature of a Dragon Break. Had Gortworg not been petitioning for recognition, Orcs likely would not have become part of the Empire.

And then the Septum dynasty ends on 3E 433, taking with it the 3rd era. Note that this means the orcs were only recognized officially for no more than 16 years. By the time of Skyrim, Orsimer has been a recognized nation for no more than 216 years. Considering that old nations such as Morrowind and Valenwood (which were officially recognized on some level long before the 2nd Era) had been taken over or ruined, it's not surprizing that the naturally warlike Orcs fought for expanding the borders of Orsinium, and having a pushback by more "civilized" races.

To me it sounded like the humans were angry for some reason and decided to take out their anger on the Orcs.