They don’t, they see how many people use the special link (Join now at www.superaids.com/yewtewber for 10% off your first order!) and if they find that fewer people are responding to the ads, the ads are simply worth less. If you skip ads without thinking, or worse with a plugin, then you won’t ever see the ads that might actually interest you, and the use of those links will decrease.
An SO will align their goals with you. A best friend may find another goal in life and go off on their own. A friend isn’t tied to you, and you’d support them if they ‘leave’ you to seek their own fortune. A partner stays with you and you and they need to find ways to reach your goals together. There’s a stability and security to a partner that you don’t get, no matter how close your friend is.