Actually not quite true. There are quite a lot of studies on chickens. Just not always designed for exactly what I might be interested in.Not too many studies on chickens. They are cheap and not kept long in a commercial setting.
Keeping a bird separate makes it easier to treat or observe. I usually leave them with the flock though.
Broilers are extensively studied for nutritional deficits and skeletal development for example.
Laying hens are used as a model for ovarian cancer in people so there are a lot of studies there.
Even behavioral studies happen albeit in a rather limited range of circumstances.
Of course most of what we want to know is still unknown - but that is true of people too!