Why no GD?

One of these may apply.
* Additional unrelated field to enter is considered unnecessary.
* Not all PHP installations support GD.

There are many third-party plugins that do just that. It’s much better to have a choice of plugins to do this rather than complicating the core code with a solution that may not be the preference of many users. The core code is set up to allow plugin developers easy ways to hook their applications into the software. In this way, you get the best of both worlds…a great blogging tool, and the opportunity to select a plugin that will meet your specific needs.

https://codex.www.ads-software.com/Combating_Comment_Spam

Specifically, https://codex.www.ads-software.com/Combating_Comment_Spam#Captcha

in brief – its not an idea that a)works all the time b) stops it all the time and c) it pisses off users of your site.