Ultimately there are too many people who have been eligible to use the lounges, who have wanted to use the lounges, relative to the size of the lounges. There’s only so much they can do to expand capacity in the airports they work in (and larger lounges are going to attract more people who still stay longer, since currently some people – like me – often stay away precisely due to the crowding).
It is simply difficult to offer a quality lounge product which includes a peaceful, quiet experience for the number of people who have access through Platinum American Express cards.