If they did that all hell would break lose. Every single facet of their appearance and behaviour would be analysed and scrutinised in the most negative way possible, and they'd be accused of being famewhores exploiting charities to try to steal attention from the other royals.
The press would delve into and try to find some way to attack the charities and private individuals associates with those charities just because of their association with Meghan (the way the tabloids smeared and tried to stoke racist hatred against the Grenfell survivors community kitchen Meghan visited, by claiming that men with "links to ISIS" had attended the same mosque that later let the Grenfell survivors use their space. There's no suspicion any of the women from the kitchen had any link with ISIS but the tabloids tried to make out Meghan was personally supporting Islamic terrorists just because she did an event in a mosque to support Grenfell!)
When Meghan privately turned up to help at the kitchens after the Uvalde shooting (with no cameras, no entourage except for one security), there were endless tabloid articles and at least four or five separate threads here absolutely ripping her to pieces for it, scrutinising her body and clothing to "prove" she had a secret hidden camera inside her top which led to fat shaming comments, and multiple posts here calling the survivors of the Uvalde massacre "liars" for posting on social media that Meghan hadn't brought a camera crew with her.
Several times Harry and Meghan have done charity stuff privately, and many months later the charity or someone from the charity has mentioned it and it's become public news, and it's resulted in the charity been deluged with hate, racist messages, and threats. More than one small charity has been forced to delete their website and all their social media just because they worked with or accepted money from H&M once, because the sheer number of abusive messages and threats was more than they could cope with.
Even here on Mumsnet, posters' hatred of Meghan is so strong, they'll happily attack parents whose children were just murdered as "liars" just for making one random comment on social media that Meghan didn't bring a camera crew.
A young woman who worked on the Smart Works launch did a podcast about her own career where she spent two minutes out of an hour-long podcast praising Meghan and saying how nice she was, there was a thread about it here that happened to be posted at 1am and within hours it had five or six pages of just non-stop mocking and abuse to the point it had to be deleted. Literally just because one person who'd met Meghan said she was nice. And really how many people are likely to be on Mumsnet at 1am, yet there were 20 different usernames all posting within minutes of each other to trash and belittle this random young woman and call her a liar. For committing the sin of saying "I met Meghan briefly and she was really nice to me."
As for Will and Kate, they do a tiny fraction the number of events that the other royals (even elderly royals) do, and routinely take several months off in the summer where they don't work at all. On average they maybe work two days a week, and that work might be a single phone call or Zoom, or something like attending Wimbledon.