I agree with KARFA’s remark that they just don’t, as a rule.

Op-ups due to overbooking are common, and the prioritisation of passengers to receive those is somewhat opaque. The passenger experience is therefore that these are ‘infrequent’. (Though I remember a lucky time where I had more than ten consecutively.)

But a plain old discretionary upgrade, other than for operational (usually overbooking) reasons, is best described as somewhere between extremely rare to vanishingly non-existent, rather than merely infrequent. I have never heard of one.