Wordpress Customizer values not show in the front page after theme installation

Question

I am working on a wordpress theme and after theme installation, the default values set for customizer are not displayed on the page, but in customizers they appear. If I change something in a field in the customizer and click publish, it is displayed on the web page. Does anyone know why? the code doesn't seem to be a problem.

customizer.php:

<?php

function photonium_customizer_register($wp_customize){
 
    $wp_customize->add_panel('home_panel', array(
        'title' => 'Front Page',
        'priority' => 1,
        'capability' => 'edit_theme_options',
    ));


    $wp_customize->add_section('home_section', array(
        'title' => 'Home Section',
        'description' => __('Here you can custom the Front Page content. <br/> To not display some fields, leave them empty.'),
        'panel' => 'home_panel',
    ));


    $wp_customize->add_setting('main_title', array(
        'default' => __('Professional Photographer'),    
    ));


    $wp_customize->add_control('main_title', array(
        'label' => 'Home Main Title',
        'section' => 'home_section',
        'priority' => 1,
    ));

    $wp_customize->selective_refresh->add_partial('main_title', array(
        'selector' => '.main-title',
    ));
}
add_action( 'customize_register', 'photonium_customizer_register' );

front-page.php:

<?php if( get_theme_mod( 'main_title') ): ?>
  <p class="main-title"><?php echo get_theme_mod('main_title', 'Professional Photographer'); ?></p>
<?php endif; ?>
...