Here's why:

* Substitution Reactions: Involve the replacement of one atom or group of atoms with another.

* Addition Reactions: Involve the joining of two or more molecules to form a larger molecule.

In esterification, a hydroxyl group (-OH) from a carboxylic acid is replaced by an alkoxy group (-OR) from an alcohol. This replacement process is the core of the reaction.

Example:

The reaction between acetic acid (CH3COOH) and ethanol (CH3CH2OH) forms ethyl acetate (CH3COOCH2CH3) and water (H2O).

```

CH3COOH + CH3CH2OH -> CH3COOCH2CH3 + H2O

```

In this reaction, the -OH group of acetic acid is replaced by the -OCH2CH3 group from ethanol.

Therefore, esterification involves a substitution of the hydroxyl group with an alkoxy group, making it a substitution reaction.