それが世の習いです。

that (indicating an item or person near the listener, the action of the listener, or something on their mind); it; that time; then; there (indicating a place near the listener); you

indicates sentence subject (occasionally object); indicates possessive (esp. in literary expressions); but; however; still; and

world; society; age; generation

indicates possessive; nominalizes verbs and adjectives; substitutes for "ga" in subordinate phrases; (at sentence-end, falling tone) indicates a confident conclusion; (at sentence-end) indicates emotional emphasis; (at sentence-end, rising tone) indicates question

as is habit; the way life normally is

be; is