Gee, maybe it's because you have a million tabs open, and all of them are websites with shitty, wasteful javascript begging the CPU for useless operations to update every single tracking pixel and cookie, with up-to-the-minute anaylitics, attached to sessions that are years old, on how your eyes move over their ad banners, so that a genetic machine learning algorithm can determine which ads are most likely to convert to clicks and then sales.

Hint: It's the ad for the top 7 ways you didn't know you could get bunions, and there's a 50% chance it will convince you to buy new shoes.