Because of the way hadith collections arose, their use as legal proof-texts by the various madahib, and the way 'ilm al-hadith developed to sort through the traditions recorded in all of these. The first collections, called musannaf (such as the Muwatta' of Malik ibn Anas), were basically transcripts of legal discourse, containing Prophetic ahadith, the reports of the Companions, the rulings of jurists, and the interpretations of scholars, all organized by specific legal topic or issue. Because of their nature as reference handbooks, these books were not concerned with proving the authenticity of the reports contained within, but merely documented the development of Islamic law as it was practiced in the early period of Islam.
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This all resulted in the situation, which persists today, that there a multiple of hadith books out there, each of which declares, on the authority of various and assorted scholars, that some ahadith are reliable and some are weak and some are outright forgeries. And they unfortunately don't all agree with each other as to which are which. As with most things in Islam, it all depends on which particular scholar you want to believe.