I explored getting a charity place in the London half marathon this morning and was very surprised to discover that none of the GiveWell recommended charities have signed themselves up as one of ~500 partners to the London Royal Parks Half Marathon this coming September. Given that >10,000 people will be participating and the easiest way to enter is to raise £350+ for a partner charity, it seems a bit odd not to be present. Given the scale of this event, I'm expecting that I'll find a similar disconnect with other major 'mainstream' events, though I haven't verified this.
I'm writing this in the hope that someone working at one of these charities can clarify the reasons for their lack of engagement with these events. I'd love to learn more and I hope to be able to raise money for your charity in connection with these events soon.