This is similar to the phrases In prison In hospital In/at church In/at school

Could be something more "legal" than grammatical in my opinion. We didn't use the definite article because we don't know which court it was. Don't lie in court, kids :)

It's because a specific court is of no concern, rather it is the category (the category of "courts") that matters. Likewise, you can say "Sam is at home" rather than "Sam is at his home" because there is no need to specify the building. Rather, it is the nature of the place, home. Likewise, you can say "I am in pain" rather than "I am having a pain" because the specific pain does not need to have an identity. Rather, it is the nature of the experience, the category of painful things.