1. not readable by other programmers (compare 1 line of code to call library function versus e.g. 15 lines of code snippet).
2. not clear (libraries have usually some API, some clear input and output, some boundaries). So bugs would be introduces easily. And you would lose more time debugging than you would save disk space... and because it would be not clear, that also would be hard to maintain, to modify such code. You would be unable to make quick changes (what if you needed to change something?)
3. When you use libraries you can always compose them, treat one library as filter for another one (for example you can use Physics Engine and connect it to Renderer). If you used only snippets you will practically not be able to make anything more complex than program that outputs Hello World 100 times using 100 different snippets...
4. libraries are usually tested by creators and if you use libraries you don't have to test library code.
But if you used snippets and mix with your own code, you would have to test these snippets also because they would be part of your codebase.
5. not properly encapsulated (unless you convert every single snippet into IIFE and inject variables to it as arguments)
All above would be obvious... And I don't believe that I have to write this down. One can argue if libraries are better than frameworks or vice versa. Or if OOP is better than FP or vice versa. Or if MVC is good or harmful. etc. But arguing that copy pasted code is better than properly separated reusable sets of functions and objects...? What year is this? 1950? ;)