If it would increase it then you wouldn't need any DEI requirements to get the employer to do it because that would be their pre-existing incentive.
Also, sometimes the data is actually available. Demographic data is generally public. If you need to hire 100 people and you start off with broad-based advertising and then after a week have filled a quarter the positions from disproportionately one area, the normal incentive is not to stop advertising in that area, but if more candidates from that area are going to mess up your numbers, now that is your incentive even if it reduces the rate at which you fill the positions.
By that same logic, nobody should ever change anything about how they do business. We were doing it, clearly it was the thing that the business chose to do, if it weren't the best thing we ever did, we wouldn't have been doing it, ergo, we should not stop doing it.
Employers make mistakes all the time. But if they're doing something against their own self-interest by mistake, you could just show them the data demonstrating their error and they would have the incentive to change it of their own volition without forcing them to under penalty of law.
The situation in question is with respect to the government, which under the previous administration was imposing these requirements on government departments and contractors by executive order.
But there have also been attempts to interpret parts of the Civil Rights Act to prohibit "disparate impact" which would effectively require DEI because neutral hiring practices could otherwise result in disparities in outcome as a result of external factors.
Also, sometimes the data is actually available. Demographic data is generally public. If you need to hire 100 people and you start off with broad-based advertising and then after a week have filled a quarter the positions from disproportionately one area, the normal incentive is not to stop advertising in that area, but if more candidates from that area are going to mess up your numbers, now that is your incentive even if it reduces the rate at which you fill the positions.