How to get media directory path in a phtml file in magento 2?

Instead of using direct object manager, use It like

use Magento\Framework\App\Filesystem\DirectoryList;

protected $_filesystem;

public function __construct(
    \Magento\Framework\Filesystem $filesystem
)
{
    $this->_filesystem = $filesystem;
}

Now you can media path by,

$mediapath = $this->_filesystem->getDirectoryRead(DirectoryList::MEDIA)->getAbsolutePath();

EDIT

If you want to use an Object Manager, then you can use this (not recommended)

$objectManager = \Magento\Framework\App\ObjectManager::getInstance();

$fileSystem = $objectManager->create('\Magento\Framework\Filesystem');
$mediaPath = $fileSystem->getDirectoryRead(\Magento\Framework\App\Filesystem\DirectoryList::MEDIA)->getAbsolutePath();
echo $mediaPath;
exit;

First you will need to inject DirectoryList class into your Magento 2 constructor:

public function __construct(\Magento\Framework\View\Element\Template\Context $context, \Magento\Framework\App\Filesystem\DirectoryList $directory_list, array $data = []) {
     parent::__construct($context, $data);
     $this->directory_list = $directory_list;  
 }

After that you will have access to DirectoryList methods for retrieving various paths. For example, to get media folder you can use:

$this->directory_list->getPath('media');

Other possible uses are:

/* Get app folder */
$this->directory_list->getPath('app');

/* Get configuration folder */
$this->directory_list->getPath('etc');

/* Get libraries or third-party components folder */
$this->directory_list->getPath('lib_internal');

/* Get libraries/components that need to be accessible publicly through web-server folder */
$this->directory_list->getPath('lib_web');

/* Get public folder */
$this->directory_list->getPath('pub');

/* Get static folder */
$this->directory_list->getPath('static');

/* Get var folder */
$this->directory_list->getPath('var');

/* Get temporary files folder */
$this->directory_list->getPath('tmp');

/* Get file system caching directory (if file system caching is used) */
$this->directory_list->getPath('cache');

/* Get logs of system messages and errors */
$this->directory_list->getPath('log');

/* Get file system session directory (if file system session storage is used) */
$this->directory_list->getPath('session');

/* Get directory for Setup application*/
$this->directory_list->getPath('setup');

/* Get Dependency injection related file directory */
$this->directory_list->getPath('di');

/* Relative directory key for generated code*/
$this->directory_list->getPath('generation');

/* Temporary directory for uploading files by end-user */
$this->directory_list->getPath('upload');

/* Directory to store composer related files (config, cache etc.) in case if composer runs by Magento Application */
$this->directory_list->getPath('composer_home');

/* A suffix for temporary materialization directory where pre-processed files will be written (if necessary) */
$this->directory_list->getPath('view_preprocessed');

/* Get template minification dir */
$this->directory_list->getPath('html');

Use below code to get the media path on .phtml file.

$this->getUrl('pub/media');

By Objectmanager

$objectManager = \Magento\Framework\App\ObjectManager::getInstance();
echo $objectManager->get('Magento\Store\Model\StoreManagerInterface')
                    ->getStore()
                    ->getBaseUrl(\Magento\Framework\UrlInterface::URL_TYPE_MEDIA);