Why do I have to add jars to class path to run the jar file when I already have the dependency added in my project?

Question

I am working on a Scala project on IntelliJ with SBT as my build tool. I started working sbt build recently. This is my project structure:

This is my build.sbt file:

name := "AnalyzeTables"
version := "0.1"    
scalaVersion := "2.11.8"

// https://mvnrepository.com/artifact/org.postgresql/postgresql
libraryDependencies += "org.postgresql" % "postgresql" % "42.1.4"

// https://mvnrepository.com/artifact/commons-codec/commons-codec
libraryDependencies += "commons-codec" % "commons-codec" % "1.13"

// https://mvnrepository.com/artifact/org.apache.commons/commons-lang3
libraryDependencies += "org.apache.commons" % "commons-lang3" % "3.8.1"

// https://mvnrepository.com/artifact/log4j/log4j
//libraryDependencies += "log4j" % "log4j" % "1.2.17"

I have Class.forName("org.postgresql.Driver) in my code to connect to database and query. Along with that, I have password decryption & logger added in the code. I am running the jar in the below format:

scala <jarname> <argument I use in my code>

The problem here is if I just submit in the way I mentioned above, I see ClassNotFoundException for postgres driver. So I add it to the classpath of the jar and submit it as below.

scala -cp /path/postgresql-42.1.4.jar <jarname> <argument I use in my code>

Now I get exception for Logger. So I add it to classpath again and it becomes:

scala -cp /path/postgresql-42.1.4.jar:/path/log4j-1.2.17.jar <jarname> <argument I use in my code>

Now I get exception for commons-codec, so I added that as well. scala -cp /path/postgresql-42.1.4.jar:/path/log4j-1.2.17.jar:/path/commons-codec-1.13.jar

Now that jar is running fine and I can see the result. So I have the dependencies added to the build.sbt file. I also did the below operation:

project structure -> Modules -> Dependencies -> + -> jars -> Add all the missing jars that are giving problems

If I remove all the -cp parameters and submit the jar with just: scala <jarname> <argument> it goes back again to ClassNotFoundException to postgres jar. So what is the point of adding dependencies on build.sbt file and then add them in the classpath again ? Is there ay setting I am missing or am I looking it in the wrong way ?

Edit: After suggestions I created a new project & copied all the code into it and added the plugin addSbtPlugin("com.eed3si9n" % "sbt-assembly" % "0.14.5") in a new file plugins.sbt file which I created in the dir /project/ as shown in the image below.

I can see the plugin in the sbt-plugins dir. But when I build the jar once again and export, it still shows 11kb jar instead of a fat jar.