I agree that understanding someone else's code is typically easier than writing that code from scratch. But that's just part of the equation. Another part is how much of that code you have to understand / write. Understanding 1 MLOC framework with all its quirks may be much harder than writing 1 thousand lines of code implementing a feature that you want.

So if you need only 5% of functionality of a huge general-purpose library, it may be still actually faster to write that 5% functionality by yourself, than to understand, tweak and later maintain the library.