First and foremost Marriott customers are not the people that book hotel rooms. Its customers are hotel owners. The loyalty program is just a direct marketing program to existing customer and Marriott has bragged about how it does it at a lower cost then competitors.
Marriott's customers, the hotel owners, are paying Marriott in part to manage the hotels but that's only a small minority of the properties. What the hotel owners are really paying the franchise fee for its to drive customers to their hotels. The Bonvoy program is a big tool for that but most Bonvoy member only do a few stays a year.
I think I a few years ago I saw a stat were over 80% of Marriott Rewards member stay less than 2 times a year and if you work out the total hotel nights vs total members that's likely about right. Marriott last year had 1.3 million total rooms world wide, thus a bit under 500 million room nights on a loyalty membership of 125 million, its 137 now but a large part of that growth is from a joint venture where its very likely few of the new member spend more than a night a year at a Marriott hotel. The article says something north of 50% of the room nights at Marriott are booked by Bonvoy members. Thus something around 250 million nights a year or around 2 nights per Marriot member per year. Given the large number of nights those that spend 50 or more nights a year spend, outside of conventions and conferences many large Marriotts regally have a third of the rooms booked by 50 night Platinum and above Bonvoy member. Its easy to see how the 80% spending less than 2 night a year is likely the case and also its likely 50% of members don't spend a night in Marriott even once every year.