Why would you not?

Libraries are more well tested. Unless you "regurgitate code in a library" as "not importing any libraries".

What value is there in tested code when you have the option of superlative, flawless code?

How do you determine flawlessness? How you even approximate a guarantee of it? To what specification is flawlessness judged, and can you precisely and completely relay that spec to your friendly local robot more efficiently than it can vendor / import existing libraries?