Because many European languages don’t have a “th” sound as in “thin” so “th” erroneously became “s” as native speakers of these European languages couldn’t pronounce a “th”.
Hence that many old English Chumoshim still transliterate with “th” as in “Sabbath” and “Emeth”.
Absence verses presence of a Dogesh should be the norm to invoke a different pronunciation as with a “Beis/Veis” and as with a “Kaf/Chaf” and as with “Peh/Feh”.