That is also supposedly the reason that Beatrice and Eugenie don't have official roles.
I think there just isn't enough work for them to be full time royals.
In recent years there's been the Queen, Philip, Charles, previously Diana, then later Camilla, Anne, Andrew, Edward, Sophie and then you have the Duke and Duchess of Kent, Prince Michael of Kent, Princess Alexandra plus the Duke of Gloucester and Princess Alice.
The Queen couldn't just say "Thanks for being there when my father (and theirs in the case of the Kent's) died young, but you'll have to find something else to do now" so there's been enough to give it until William and Harry were adults before they needed to start pitching in.
Once the Queen, Philip, the Kent's and the Gloucesters aren't around there will still be Charles, Camilla, William, Catherine, Harry, Meghan, Andrew, Anne, Edward and Sophie, which will be plenty with a lesser workload than what there used to be. In 20 years time when it's time for the older royals to retire/slow down then the current royal kids will be old enough to start stepping up (and the age difference between Charles and Edward will help cover that transition).