osCommerce and Drupal have vibrant development communities. As well as their core development teams, each CMS has hundreds of members all over the world who constantly produce new modules for the standard installation. These modules are called various things, such as addons, plugins, components, mambots, and extensions. We will call them all "extensions". Extensions do not come with the standard install because most of them are not needed by most people. You can imagine that if even a small selection of the thousands available were included, they would make the initial installation far too large.
The vast majority of the these extensions are free to download and install and easily available from the relevant websites (see Further Info links). Some must be paid for but at very low cost considering the extra function they provide.
To find out what extensions are provided by each community you should visit the Drupal and osCommerce websites.