Here's why:
* The vowel "o" is followed by a consonant ("s"), which typically indicates a short vowel sound.
* The vowel "o" is not in a closed syllable (a syllable ending with a consonant). Closed syllables often have long vowel sounds, but "loser" is a single syllable.
Think of the short "o" sound in words like "hot," "lot," and "dog." The "o" in "loser" sounds similar to these words.