I think there is no need to include composer for loading any library etc. Loading some libraries is not hard task. Its easy to include...
Download library > extract > put into your project > use it
This task is only one time per project. I mean its not over and over again.
Updating is not huge task too... just download and replace with old files in your project.
this is just 30 seconds for each library. I do not see any reason to include one more layer (composer) to my project.
Plus if we add this layer (composer) to our project, it brings its own problems. Check this to see how many people dealing with composer errors; https://stackoverflow.com/search?q=composer+error
I think I overlook something here. Because almost everybody use composer and recommend.
Could you tell me what I do overlook about composer? What are the benefits of it?