I bet the reason for this is that bulk ad sales are generally more profitable than selling individual ads. Advertising companies prefer that advertisers do not

I bet the reason for this is that bulk ad sales are generally more profitable than selling individual ads. Advertising companies prefer that advertisers do not become accustomed to purchasing ads on a one-off basis. Selling ads in bulk is much more lucrative for platforms like Google Ads. It is simply easier and more profitable for these companies to sell a large volume of advertisements rather than just one ad at a time. Ultimately, there is little profit in creating a system where ad viewing is optional. Advertisers also do not want this because a lost ad viewer could potentially become a lifelong customer. Essentially, they could lose a customer over a few cents saved or added revenue. Remember that we are the product, not the customer.