What is one reason why it it often hard to distinguish between offensive and defensive players?

In some sports there is no difference between offensive or defensive.

Explanation:

In games like tennis, even when it's double tennis so it's two people, the players are neither one nor the other. In sports without set roles such as football or rugby for example, it's not possible to constantly be only offensive or defensive.

But also, players can, depending on the sports, switch between offensive and defensive easily.

In football you get set players with set roles in where to play, especially because it's a team sport.

And a lot of the time the shirts aren't labelled with those two words on them either. So it's a very vague question. But I would mostly just say that in a lot is sports there is no difference.

the members of alcoholic anonymous aren't required to disclose their last names.