Why does doubt not nullify prayer, while it does nullify ablution, as is established in books of Fiqh (Islamic jurisprudence)?

The majority of scholars hold that a transient doubt regarding purity does not affect it, whether it occurs during prayer or outside of it, and they cite the hadith: "He should not leave until he hears a sound or perceives a smell." The Malikis differed from the majority, holding that doubt about purity nullifies ablution as a precaution for prayer. In one narration from them, they distinguished between doubt during prayer and outside of it: doubt during prayer does not invalidate it because the original state (of purity) is certainty, whereas doubt outside of prayer necessitates renewing ablution because entering prayer must be with certain purity.