One wonders, why give even just George the title, why not wait until Charles is king? I don't know but I'd guess it's to demonstrate that he is third in line to the throne. So in that case, why not give the title to his siblings, to demonstrate their position in the line also?
Not that I'm a monarchist, but to me it seems sensible to restrict the titles and 'working' royals just to those in the direct line to the throne, plus their spouses.
The notion of the 'spare' really is anachronistic in this day and age - realistically how often is the 'spare' likely to need to step in, and even if they had to, to do so from a broadly normal life would be rather refreshing.
For several generations now the 'spare' has been lost in terms of role and has proved problematic to the royals - Margaret, Andrew, Harry. So much better right from the get-go to make it clear to the 'spare' that they wont be a senior working royal and they'd be expected to make their own way in life.
Now it is too late for William & Harry's generation, but not so for the next generation down. But this would mean no titles for Charlotte and Louis and them being brought up to recognise that they wont be senior working royals and will need to live a basically normal life (or at least as normal as a royal can). Now although they look high up in the line to the throne, they'll drift lower as George has kids. Let's not forget that we now see Margaret (before she died), Andrew and Harry as being no-where near to the top of the tree yet they were once 2nd, 2nd and 3rd in line.
It is the notion of the 'spare' which has been really problematic for the royals over generations but they seem not to learn as they seem to be making the same mistake for a further generation in not recognising that that Charlotte and Louis are just Margaret, Andrew and Harry but just in a different generation.