Delivery by meteorites isn't necessarily the only or even primary source, but the fact is that it is a source we can directly observe, and it is a sufficient source. Earth's primordial chemistry likely did create more, but the existence of those conditions is theoretical, and such additional production need not be invoked to explain the needed level of abundance. Even a world with very different conditions from primordial Earth would likewise receive vast quantities of nucleotide during its formation and continuous replenishment afterwards.