Being wealthy doesn't always mean having something in common enough to interact with Hollywood celebrities, but being a wealthy Scientologist can. A wealthy Scientologist can take courses at a Celebrity Center, and attend parties and events with other celebrities, and feel they are part of something bigger (Scientology -- saving the planet) that gives them their common ground. Not only that, they feel like they've become celebrities themselves, at least within the cult.
For example, let's say Mrs. Duggan is starstruck but finds that associating with other super rich people can be boring, and dreams of being in a group where their money would have this pretend significance of saving the planet, yet allow them to associate with some Hollywood stars, people she considers to be really "important."
Let's say you want to be in therapy and can afford the best shrink in the world. That isn't going to give you opportunities to meet stars.
Or, let's say Bob Duggan starts backing Tom Cruise's new movie(s) heavily, and then the Duggans may get to meet Cruise or hobnob with him more, but that would make them feel like buying in.
Also, being rich doesn't necessarily make you a celebrity and important, but people feel that belonging to the "Celebrity Center" can do that for them. Plus, spiritually, they feel like they may have gone from fucked up or confused rich people to mankind's greatest hope, big beings.
Smaller whales feel that it will help their businesses, and for some it may do that. They like the idea of feeling special and being "celebrities" too. This is a rather unique thing to Scientology, the Celebrity Center thing -- being confirmed a "celebrity" just by having enough money to attend the Celebrity Center.
As others have mentioned, whales may have businesses that are enmeshed with other Scientologists, and dependent on them.
About the associating with celebrities alone, one might question, well, why should that keep them in Scientology -- it gets them in but why should it keep them in?
Because they know if they leave, they will not be a celebrity anymore. Instead they would have to face the embarrassment that they were rich dupes who fell for a scam, and who had let their vanity be manipulated. That's too much for them to face.
This isn't to downplay the personal dirt that may be in their auditing folders though. That could be enormous. Just the stuff whale husbands and wives may not have even told each other but is in those folders is enough to keep some of the rich couples in, when one may feel like it was time to leave.
Another problem is that wealthy people sometimes live in a bubble created by their wealth, that the lower level clams may not have. People they come into contact with outside the cult will not tell them anything straight about Scientology, ever. I don't think its as much of a bubble as Hollywood actors are in, but it's still there.
For example, certainly there's got to be one top executive at Duggan's pharmaceutical company who knows that Scientology is a farce, but he's not going to mention that to Bob, or anyone at the firm, because it would mean risking his job.