A lot of people don't miss the person after a breakup, they miss the attention, validation and routine.
I think many people confuse emotional withdrawal with genuine heartbreak. After a breakup, people often say they "miss their ex" but if you really examine what they miss , it is not really the individual person and more what the relationship provided psychologically. They miss having someone text them every morning, someone to vent after a bad day , someone who made them feel wanted, attractive or emotionally safe.
The breakup creates an emotional vacuum , and people interpret that emptiness as missing the person themselves. This becomes even clearer when you notice how quickly some people move on once another source of attention or emotional comfort appears. If the connection was truly about the individual , replacing them emotionally wouldn't happen so fast. You get attached to the habit of being loved , the companionship and the validation that someone chooses you every day . So losing that would obviously feel like losing a part of yourself. Modern dating culture intensifies this , people are constantly seeking reassurance, emotional stimulation and validation. Relationships become less about deeply understanding each other and more about avoiding loneliness and feeling emotionally affirmed. That's why some people even miss their toxic exes, because even unhealthy attention can become psychologically addictive .
So people aren't grieving the person they are grieving the role that person played in their life . Thus if they find someone else who can fulfill that role the memory of the ex they cried about is completely gone .