Python の Caesar Cipher が最後にシフトされた文字のみを返すのはなぜですか?

<code class="python">plainText = input("What is your plaintext? ")
shift = int(input("What is your shift? "))

def caesar(plainText, shift):
    cipherText = ''
    for ch in plainText:
        if ch.isalpha():
            stayInAlphabet = ord(ch) + shift
            if stayInAlphabet > ord('z'):
                stayInAlphabet -= 26
            finalLetter = chr(stayInAlphabet)
        cipherText += finalLetter
    print("Your ciphertext is: ", cipherText)
    return cipherText

caesar(plainText, shift)</code>