Nearly all bugs in anyone's code come from things you get right 99% of the time, but mess up that 1%. If Ruby's standard library provides a correct implementation of `Array#reject!`, it's eliminating one thing, however insignificant, that you might mess up. That even standard library implementers occasionally mess up iterating over a dang array (`#reject!` was correct but inadvertently suboptimal) is a good example of how, in the large, "just get it right" is much easier said than done.
Another good reason is that the Ruby standard library implements many methods, such as `#reject!`, in C instead of pure Ruby, for vastly better performance. You can do that yourself of course, but that sort of defeats the purpose of using Ruby. Try implementing some `Array` iteration methods in pure Ruby with for loops, then benchmark them against the standard equivalent; they'll usually be dozens to hundreds of times slower.
Another good reason is that the Ruby standard library implements many methods, such as `#reject!`, in C instead of pure Ruby, for vastly better performance. You can do that yourself of course, but that sort of defeats the purpose of using Ruby. Try implementing some `Array` iteration methods in pure Ruby with for loops, then benchmark them against the standard equivalent; they'll usually be dozens to hundreds of times slower.