Why does French verse count syllables when English verse counts beats?

Because the two languages put their rhythm in different places. English has a strong stress accent, so English verse arranges stresses. Modern French does not have a significant stress accent, so the French metric line is generally determined by the number of syllables. Syllabic verse is common in syllable-timed languages such as French or Finnish, while stress-timed languages such as English favour verse that counts stresses.

The great French line is the alexandrine, twelve syllables. Its foundation is two half-lines of six syllables each, separated by a pause, a caesura, with a stressed sixth and twelfth syllable. The name comes from the medieval Roman d'Alexandre of 1170, though the line had been used a few decades earlier. In time it became the preferred line for the prestigious genres of epic and tragedy, and classical alexandrines are always rhymed, often in couplets alternating masculine and feminine rhymes.

Counting has its own French rules. A silent or mute e counts as a syllable before a consonant but not before a vowel, and at the end of a line it falls outside the count. Why six and six? Humans can perceive the number of items in a small set, up to about five to nine, without counting, so half-lines tend to be short and are marked off by stress, word boundaries and rhyme.

English poets keep their distance from this. English listeners tend to perceive stresses at equal intervals, so verse counting stresses is heard as rhythmic, while verse counting only syllables generally cannot be told apart from free verse. In English, the alexandrine is a rare variant: where other literatures made it their heroic line, English is a notable exception.