>It's because English has no (or very few - I can't think of any) words that begin with the same phoneme.

Loan words, but: Tsar (zar or sar), Tswana (50/50), and Tsetse fly (usually /ts/) from the Tswana language. I don't think /ts/ ever refers to something specific in native English, it's usually plurals like it-s or from suffixes like bet-sy, gats-by, wat-son.