ON CONFLICT clause is not supported with partitioned tables

[farons]

postgres 12.5
An error occurred when I tried to use upsert to update my main table；
but when I use the traditional way:
CREATE TABLE o(id INT PRIMARY KEY, i INT) PARTITION BY RANGE (id);
CREATE TABLE o1 PARTITION OF o FOR VALUES FROM (1) TO (1000);
CREATE TABLE o2 PARTITION OF o FOR VALUES FROM (1000) TO (2000);

INSERT INTO o(id, i) VALUES (1,1),(2,2),(1500,1500);

INSERT INTO o(id, i)
VALUES (1500, 1400), (2,20), (3, 3)
ON CONFLICT (id)
DO UPDATE SET i = EXCLUDED.i; it is ok.
why?