why plugins are loaded prior to pluggables

Directly from source for pluggable.php:

<?php
/**
 * These functions can be replaced via plugins. If plugins do not redefine these
 * functions, then these will be used instead.
 *
 * @package WordPress
 */
?>

So, there’s your answer, the functions in pluggable.php are intended to be overridden by Plugins.

Re: load order:

See this post by Konstantin Kovshenin. The relevant points (10-15):

1. wp_get_active_and_valid_plugins() retrieves the list of all active plugin files for loading and includes them. This is the point where your plugin code gets executed, functions, classes defined, etc.
2. Includes wp-includes/wp-pluggable.php and wp-includes/wp-pluggable-deprecated.php which include functions (and deprecated functions) that can be redefined by plugins. Like wp_mail() for more advanced mailing, wp_authenticate() for alternative authentication methods, etc.
3. wp_set_internal_encoding() is called to set the internal encoding according to the blog_charset option.
4. wp_cache_postload() is called if object caching is enabled.
5. At this point a plugins_loaded action is fired. This is the very first action (after muplugins_loaded fired before loading the non-multi-site WordPress plugins) that you can hook into, it comes before the init because WordPress has not been initialized yet, at least not fully.

So, simplified:

1. Plugins are loaded
2. pluggable.php is loaded
3. plugins_loaded action is fired

Which is the expected order.

(Note: this all takes place in wp-settings.php.)