Because when the lady undresses, then she handles her clothing, rather than her tits. She pulls off and manipulates her clothing and does almost nothing with her tits. But when the lady pulls out her tits,
then she mainly handles her tits, and her clothing is a secondary thing. The main action is with her tits and not her clothing.
Perhaps some porn-fans have a fetish for women's clothing, and they want to see the woman handle her clothing rather than her tits. But think this is a niche thing. Most porn-fans probably find it hotter to see the lady handle her tits rather than her clothing.
Here are some examples, where the lady pulls out her tits:





And here are some examples, where the lady pulls off her clothing, without doing much with her tits:

