Sorry who reports on the the Royals doing charity work?
That's what the Royal Rota exists for.
It's not a secretive group that gets priority briefings. It's a pool arrangement, sharing stills and video plus 1x print reporter, who go along (in turns, by rota) to cover engagements when either the venue is too small for a full press pack or when the engagement might not otherwise garner much attention. The rota output is pooled, so any outlet has it available and can choose to run it. Whether they do or not is up to individual editors, and depends on what pool material there is and what other news there is that day. MSM is not a monolith, and those decisions will vary.
And yes, there is reporting. If you google then you find things like "Princess Alexandra visits market in Brixton" which I very much doubt was covered other than by the rota
It's a shame that some work does not get more attention (perennial example is the Duchess of Edinburgh's work against VAWG, FGM and rape as a tool of war)
But she carries on with it, irrespective of British tabloid interest. I don't think she's chasing press attention to validate it I think she's doing it because she thinks it's right. And instead of attention, she gets international respect, especially when she takes those messages on her visits to places like Colombia and Sudan.
There is of course also the late Queen's maxim that she had to be seen to be believed - ie that they have to be out and about. And I think they do that because it's valuable in itself, irrespective of whether it makes the press. And of course you're right in the sense that only a tiny fraction of eg the Princess Royal's engagements ever get coverage. But she still keeps going, without any signs whatsoever of courting the press.