Why is Qasamah considered weak evidence?

Primary text

Qasamah is considered weak because it is established by the claimant's word and oath, despite the suspicion regarding the claimant's truthfulness and the existence of enmity that would prevent the validity of testimony establishing a right for another. Therefore, it is even more appropriate to reject the claimant's solitary word in establishing a right for themselves. This contrasts with clear testimony (bayyinah), which is strengthened by the number and justice of the witnesses, the absence of suspicion regarding them (as they establish no personal rights or benefits, nor avert harm from themselves), and the absence of enmity towards the accused. This strength allows clear testimony to establish all rights and prescribed punishments (hudud) that are otherwise negated by suspicion.