Problem with Databaseseeder

You see, I have the following tables:

Schema::create('empresas', function (Blueprint $table){
        $table->increments('id');
        $table->string('nombre');
        $table->timestamps();
    });

    Schema::create('coches', function (Blueprint $table){
        $table->increments('id');
        $table->string('nombre');
        $table->unsignedInteger('empresa_id');
        $table->foreign('empresa_id')->references('id')->on('empresas');
        $table->integer('precio');
        $table->timestamps();
    });

So, I prepare the following in DatabaseSeeder.php:

public function run(){
    $a=DB::table('empresas')->insert([
        'nombre'=>'Ford'
    ]);

    $b=DB::table('empresas')->insert([
        'nombre'=>'PSA'
    ]);

    $c=DB::table('empresas')->insert([
        'nombre'=>'Volkswagen'
    ]);

    DB::table('coches')->insert([
        'nombre'=>'Alfalfa Romeo',
        'empresa_id'=>$a,
        'precio'=>54000,
    ]);

    DB::table('coches')->insert([
        'nombre'=>'Bionic Diesel',
        'empresa_id'=>$a,
        'precio'=>27000
    ]);

    DB::table('coches')->insert([
        'nombre'=>'Sitsume Kaso',
        'empresa_id'=>$b,
        'precio'=>45000
    ]);

    DB::table('coches')->insert([
        'nombre'=>'Audi Fono',
        'empresa_id'=>$b,
        'precio'=>59600
    ]);

    DB::table('coches')->insert([
        'nombre'=>'Renault Polo',
        'empresa_id'=>$c,
        'precio'=>50900
    ]);

    DB::table('coches')->insert([
        'nombre'=>'Canis Lupus',
        'empresa_id'=>$c,
        'precio'=>49900
    ]);
}

This should create 3 companies and 6 cars, assigning every 2 of these a company, but I find the following:

They all have the id of the first company! How do I solve this?