Only a few people don’t follow their ex-partner’s page after a breakup.
Even those who deny it most likely visit their ex-lover's page at least once a month.
This is a completely normal phenomenon if the past relationship meant something to the person.

But there are several reasons why people do this.
Supporting self-esteem
One of the common reasons is related to trying to find even the slightest evidence that the ex-partner is unhappy after the breakup.
Each person sets their own criteria for confirming or refuting their guesses.
The point of such behavior is to try to maintain self-esteem.
Nostalgia
This motivation usually occurs a considerable time after the breakup. Who doesn't like to indulge in nostalgia?
Curiosity
This is also a popular reason why a person spies on an ex-partner's account.
First of all, you need to check whether this person has found a new love, how he spends his time, what’s new with him.
Hope dies last
Sometimes it happens that a person wants to restore a relationship, but does not know where to start.
Monitoring your ex's social media helps you work out tactics and find something to latch onto. Whether the attempt will be successful is a separate question.