The reason phone vendors can advertise capacity is because the load (the phone) is a known quantity. They made the phone, so they can reliably estimate the battery’s capacity based on average use by that phone.

Similarly, power bank manufacturers can do the same, because the load is controlled by them. The USB port might only provide 5V at 1.5A or 3A - whatever the power bank manufacturer put in - so they can reliably estimate how much current over time the battery can provide.

But makers of alkaline batteries don’t have that knowledge. They have no way of knowing if you’re going to put them into a kid’s toy that pulls only 20mA, or a DC motor for a rotisserie that pulls 1A. So they can’t possibly provide you any measure of Ah that is going to satisfy all consumers. If they did, they only open themselves up to legal problems for making misleading claims about their product.