Really interesting to see the cost breakdowns above. In answer to the OP, it’s basically because staging events isn’t cheap, especially as they scale.
A very tiny event can run with volunteers. A slightly bigger one might be able to break even with volunteers plus a single figure £ entry per rider making just enough to cover, say, equipment hire and any venue fees (etc.). A much bigger event will have large production requirements in terms of time, and won’t run well at all without at least some paid staff. As production time reaches further back from the event day, and staffing requirements scale beyond what a small group of volunteers can do or coordinate, that starts reaching into significant costs. Infrastructure and venue fees don’t necessarily benefit from any economies of scale either.
The frustrating thing as an organiser (I don’t run bike events, but I do run events) is that what you and your team do is absolutely exhausting, often involves long hours and a whole load of crisis management, yet is essentially invisible to everyone who turns up and makes use of all the stuff you sorted out. I think that’s one of the reasons events can feel expensive.
In that SDA list above I’d query what the following are for:
NON-RACE COSTS £16,117
PRIZES/TROPHIES £12,380
MARSHALS £8,190
They’re laughing at you right now from their cigar and champagne den.