The story
Because the two "weak" vowels, /ə/ and /ɪ/, which are often found at the end of non-ultimately-stressed words, are not distinguished.
Etymology from Wiktionary, CC BY-SA 4.0.
weak vowel merger
Today it means: A phenomenon found in English pronunciation where the phonemes /ə/ (schwa) and /ɪ/ (the near-close near-front unrounded vowel) are not distinguished from eachother when unstressed, with the resulting ambiguous phoneme represented in the International Phonetic Alphabet as /ɨ/ (a close central rounded vowel). Full definition →