Plenty of people have donated huge amounts of money.
The screenwriter James Graham (who was one of the people instrumental in drafting and negotiating the £1.57 billion bailout) donated his entire salary from his last TV series to a charity that financially supports arts freelancers who have lost their jobs. It got barely any media attention.
Andrew Lloyd Webber, Benedict Cumberbatch and Sophie Hunter, Imelda Staunton, Eddie Redmayne, Sonia Friedman, Colin Firth, Hugh Bonneville, Tom Hiddleston, Michaela Coel, Michael Frayn, David Hare, Nicholas Hytner, Emma Thompson, David Walliams and Edgar Wright all donated large sums to the Theatre Artists Fund. Again, little media attention.
Tom Hiddleston and his family are huge theatre donors and his mum in particular has worked incredibly hard and given so much of herself over the years to support theatre artists who are in the early stages of building their careers, including myself. Few outside of the theatre industry know that. I don't know what Tom and his mum have been doing during this crisis (apart from the above donation) but from my personal experience with both, I'd be astounded if they hadn't been heavily donating and most likely getting personally involved too.
Benedict Cumberbatch donated £5k just to one person's kickstarter, and it's only public knowledge because the person went to the press. That's just one tiny individual kickstarter. If he donated that much to a single person, it's reasonable to assume he's been making more donations and likely larger donations to theatres and arts charities.
James McAvoy donated £30k to one charity that I know of.
And most of these donations have been to charities directly supporting workers who have lost their entire income overnight, not theatres themselves. In times of crisis people take priority over buildings.
It's a huge scandal how freelancers who have lost their entire industry overnight have been left with nothing except their savings (if they have any) and whatever they can earn from finding a new job in a supermarket or as a cleaner (if they can). Most freelancers are not eligible for the SEIS. I have friends who earned a decent amount, who were told they were only eligible for ludicrous sums like £8. Friends who are disabled who have been waiting since May for PIP decisions.
The potential loss of buildings is serious, but people are more important. This industry is facing a mass talent exodus as tens of thousands, maybe millions of people both in creative roles and in technical roles are being forced to leave the industry. Many will never return. When and if this pandemic ends, we will be seeing the repercussions for decades in the dearth of talent and especially the absence of actors and writers from less wealthy backgrounds.
The focus on helping actual people coupled with the sheer scale of money needed to keep theatres and theatre companies (or other arts companies) from going under, means it's just not practical to rely on individual donations even from wealthy people.
Plus most people working in theatre are not wealthy. The handful who are (like Andrew Lloyd Webber and Cameron Mackintosh) are actively involved in the fight to save the arts. Most actors, even quite famous actors, aren't super-wealthy. The 1% of Hollywood megastars bringing in big money,often don't come from a theatre background. And it's not like theatre is collapsing in isolation. There are so many 'causes' which need help, some of which people have personal connections to. For example I have one fairly famous friend who has a child with Down's Syndrome so while she wishes she could do more to support the arts, she's prioritising giving her time and money to charities relevant to her child.
There are just too many parts of society that are collapsing which need money, and only proper government support (not just financial support, but being given clear guidance with which to make plans) can help.