I am guessing that true people pleasers may not understand this fully themselves. But I would think that it traces back to childhood whereby the person didn't receive approval or validation from a parent, so they aim to please everyone out of fear of being rejected. A lot of people people please, in fact.

Why do people pleasers need approval?

"Poor self-esteem: Due to a lack of self-confidence, people-pleasers struggle with validating their own desires and needs. They therefore seek external validation to gain approval and acceptance."

The following factors might play a role in the development of people-pleasing traits:

• Childhood Trauma: Difficult or traumatic past experiences, such as abuse, can lead people to become more agreeable in order to feel safe and secure. They may believe that pleasing others will stop any abusive behaviors from being triggered.
• Family Dynamics: People-pleasing is frequently a learned behavior. Children of people-pleasers may have witnessed their parents’ validation from certain behaviors and learned to act accordingly.
• Poor self-esteem: Due to a lack of self-confidence, people-pleasers struggle with validating their own desires and needs. They therefore seek external validation to gain approval and acceptance.
• Perfectionism: People-pleasers can often exhibit perfectionist traits which lead to a need for control over how other people think and feel.

Source: The Disease to Please: Hypervigilance Around Others’ Needs - Heather Hayes & Associates

You can always make other plans if someone is dragging their heels on making a plan with you. Then when they ask, just say, sorry, I made other plans since you couldn't commit. It's Ok to do that. Or, don't try to make plans with someone who persistently cannot commit.