Rewritten rows on unchanged values

I noticed how rows were re-written to a different location (new ctid) even
without changes to the values. This illustrate what I mean:

CREATE TABLE demo (id serial, value text);

-- generate a few pages of dummy data
INSERT INTO demo (value)
SELECT md5(s.a::text)
FROM generate_series(1, 1000) AS s(a);

-- ctid = (0,1)
SELECT id, xmin, ctid, value
FROM demo
WHERE id = 1;

UPDATE demo
SET value = value
WHERE id = 1;

-- ctid = (8,41)
SELECT id, xmin, ctid, value
FROM demo
WHERE id = 1;

I'm curious as to what would prevent keeping the row where it is and maybe
change xmin in place?

Thanks,
-b