@nidupb Welcome to the Netlify community.
I’m with you. I don’t like trusting important tasks to black boxes. I think the main reason things are done this way is so that teams can work on the same project via GitHub (etc.), or you can work on the same project from various devices, locations, etc., also via GitHub. The secondary benefit is that building the full site usually more than duplicates the space requirements on your drive(s) – including your backups.
However, there is no law that says you cannot upload / post only the built files yourself. In fact, that’s what I do and it works great.
The caveat is that you may be “limited” to drag-n-drop updates. They run like a scalded cat, but you do have to ZIP up your project, open the Netlify dashboard for the site, and so on, whereas with GitHub integration you simply push your updates and they appear on Netlify.
Maybe someone brighter than me can tell you how to incorporate GitHub with your build folder only, but my understanding of the way that Pelican and Nikola and the others work is that they can (at least sometimes) clear out the build folder, which could mean clearing out the .git directory, which would mean losing that project’s connection to GitHub … and that would be a pain in the neck if it happened often.
However, if you want to use a build tool to generate your site, and you don’t mind the steps involved with drag-n-drop, you’ll be astonished at how quickly Netlify deploys your project.