Everybody that claims swine are filthy animals: that is absolutely not true.
They are filthy because they have to sleep in their own shit, because the farmer presses them together in such a small room that they literally cannot move without hitting each other.
If pigs are raised on an open field, they roll themselves in water to keep them clean multiple times per day. On the other hand cows roll themselves in their shit, even when provided with puddles of water...and are unable to clean themselves afterwards because they ain't gonna take a bath.
Source: me, my grandparents had a rather mid-large farm in Germany and tried to raise their stock as humane as possible.
They also tried to raise chicken in an open manner, but that's close to impossible because you would have to have a lot of bushes, puddles, mixed areas of grass, plants, and a lot of fences in between (so groups don't attack other groups) around the stable. Otherwise chicken won't leave the stable, even not when they get infected.
And the value of chicken stock is so low that it's a financial disaster if you would build up a healthy environment for them, even if you would own the land. They would just destroy the plants so regularly that you would have to replace the plants at least on a monthly basis to provide hideouts and shadow for them.