Because they are alotted a finite amount of time to speak and/or question someone and they feel they have more to gain by using their time on the topic at hand. Often there is a bit of coordination as to who asks what and who makes statements thus sharing their time getting their points across.

The antagonists know this and use their time to goad their opponents into wasting their time rebutting or defending their point against their outrageous bullshit.

Many see it as a game, it's not, but that's how they roll.