Why do people speed up when you put your turn single on?

I know when you're merging each vehicle is required to let one person over, but I don't know if that applies to just a lane change. Annoying af. Sometimes I have to stay in the left to get on the freeway, because even though I'm going 10 plus over the speed limit some asshole is still going to speed on past me as soon as i put my turn indicator on.

Why do people speed up when you put your turn single on?
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