Why do scholars use the phrase "it is not sound" (لا يصح) instead of "it has not been proven sound" (لم يصح) when ruling on the authenticity of a Hadith, which causes confusion for the questioner?

There is no cause for confusion. Many of those who speak on this matter say "لم يصح حديث" (a hadith (definition) has not been authenticated) instead of "لا يصح حديث" (a hadith is not authentic), because the word "لا" (la) is used in theArabic language in place of "لم" (lam) and "ليس" (laysa), as Al-Khalil ibn Ahmad al-Farahidi definitively stated in his book "Al-Jumal," saying that "لا" (la) comes with the meaning of "لم" (lam), as in Allah's Almighty saying: "فلا صدق ولا صلى" (So he neither believed nor prayed) meaning "لم يصدق ولم يصل" (he did not believe and he did not pray).