Just because that is the poetic form in English. Haikus vary in syllables sometimes, because the form comes over from another language, but 17 is the standard.
"Seventeen Syllables in a Haiku Poem". 17 = Syllables in a Haiku Poem
Haiku
17 syllables in a haiku poem
haiku is a Japanese form of poery that contains only 17 syllables.
haiku
"Haiku"
A haiku is a short Japanese poem with 17 morae, similar to syllables, in lines of 5, 7, and 5.
A traditional Japanese haiku has three lines. The first and third lines have five syllables, while the second line has seven syllables. They do not usually rhyme but on can. As the short answer to your question there are seventeen syllables.
haiku
A haiku on rainobtained from the internetain't original:)
in second line of a haiku there is 7 syllables
First line has 5 syllables Second: 7 Third: 5