Why Drupal doesn't use more Pear librairies ?

Hello,

I am new in drupal and i am still trying to have a global view of all things provide by your nice CMS. I found many thing which exist both in Drupal and Pear. Apparently, it seems that Pear librairies are more sophisticated than Druapl one (form_builder, Dataobject, html_table, ...?). In another hand when i installed drupal, some Pear librairies were required. So my question is why drupal doesn't make a more extend use of pear librairies ? This could make module developpement more easy, fast and with more option, and larger developper communauty !