Because A level predictions are highly unreliable and often optimistic. Statistically a high proportion of predicted A levels won’t be met. A study from 2020 looking at 3 years of data across all university applicants found that only 16% of applicants’ predictions are accurate and 75% of applicants do less well than predicted.
The gap between prediction and reality is less bad at Oxford than in the sector more widely, partly because Oxford does a lot of extra tests and assessments and partly because Oxford applicants are, as you say, already high achieving and often predicted higher grades than their offer conditions (so they can afford to slip a bit). But even so if Oxford colleges are making hundreds of offers, statistically some people will miss their grades. Oxford could just leave those places unfilled but given there are more talented applicants than places available, it is a shame not to fill them and that is the purpose of the open offer scheme.
The basic problem is that having people apply to university before they have their grades is a stupid system but there is no political will to change it.