spring-boot-maven-plugin doesn't create fat jar

it seems you are using a wrong command. mvn clean package is maven command, you should use command 'repackage', it used for

Repackages existing JAR and WAR archives so that they can be executed from the command line using java -jar

as it mentioned here https://docs.spring.io/spring-boot/docs/current/maven-plugin/repackage-mojo.html

Or probably it's plugin configuration issue. Just checked: it works with spring-boot-maven-plugin-2.0.0.RELEASE

<plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
    <executions>
        <execution>
            <goals>
                <goal>repackage</goal>
            </goals>
            <configuration>
                 <classifier>exec</classifier>
            </configuration>
         </execution>
    </executions>
</plugin>