为什么 Maven 发誓？javax.servlet 不存在

在构建时，maven 发誓：

import javax.servlet.ServletException; //package javax.servlet does not exist
import javax.servlet.annotation.WebServlet; //package javax.servlet.annotation does not exist
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

而且不仅在这些。

这是为什么？简单的开始，一切正常，但是在构建maven的时候，有什么问题呢？这些库是通过“项目结构/库”添加的。

更新： pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
         http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>groupId</groupId>
    <artifactId>WebBD</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>jar</packaging>

    <properties>
        <tomcat.version>8.5.23</tomcat.version>
    </properties>

    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>javax.servlet</groupId>
                <artifactId>javax.servlet-api</artifactId>
                <version>3.0.1</version>
            </dependency>
            <dependency>
                <groupId>org.apache.tomcat.embed</groupId>
                <artifactId>tomcat-embed-core</artifactId>
                <version>${tomcat.version}</version>
            </dependency>
            <dependency>
                <groupId>org.apache.tomcat.embed</groupId>
                <artifactId>tomcat-embed-jasper</artifactId>
                <version>${tomcat.version}</version>
            </dependency>
            <dependency>
                <groupId>org.apache.tomcat</groupId>
                <artifactId>tomcat-jasper</artifactId>
                <version>${tomcat.version}</version>
            </dependency>
            <dependency>
                <groupId>org.apache.tomcat</groupId>
                <artifactId>tomcat-jasper-el</artifactId>
                <version>${tomcat.version}</version>
            </dependency>
            <dependency>
                <groupId>org.apache.tomcat</groupId>
                <artifactId>tomcat-jsp-api</artifactId>
                <version>${tomcat.version}</version>
            </dependency>
        </dependencies>
    </dependencyManagement>
    <build>
        <finalName>embeddedTomcatSample</finalName>
        <pluginManagement>
            <plugins>
                <plugin>
                    <groupId>org.codehaus.mojo</groupId>
                    <artifactId>appassembler-maven-plugin</artifactId>
                    <version>2.0.0</version>
                    <configuration>
                        <assembleDirectory>target</assembleDirectory>
                        <programs>
                            <program>
                                <mainClass>Main</mainClass>
                                <name>webapp</name>
                            </program>
                        </programs>
                    </configuration>
                    <executions>
                        <execution>
                            <phase>package</phase>
                            <goals>
                                <goal>assemble</goal>
                            </goals>
                        </execution>
                    </executions>
                </plugin>
            </plugins>
        </pluginManagement>
    </build>
</project>