I am still surprised the situation regarding titles for female line grandchildren wasn't reformed at any point in the last 60 years.
I find your comment here particularly entertaining given your argument that the monarchy needs to be downsized under Charles.
The BRF isn't known for being progressive - monarchies as a whole tend not to be - and thus typically makes changes in response to a current need instead of a potential future need. There hasn't really been need in the past 60 years to address the titles of the female-line grandchildren.
During George VI's reign the issue was addressed when the then Princess Elizabeth was pregnant with her first child and LPs were issued to create any children she has royals. Similarly, at her marriage her husband was created an HRH, if not actually a Prince. George's other daughter didn't marry or have children during his lifetime, so we can't say that he didn't intend on doing similar for her when it came up - reactionary, instead of progressive.
When Margaret did marry and have children it wasn't decided that her husband or children needed to be royals - although her husband was raised into the nobility, to give him an equal status to previous husbands of a daughter of the monarch. Given just who Margaret was and elements of her personal life, I would think that making her husband and children royals wouldn't have been a good long term investment for the BRF.
Of the Queen's children only one of them was female - meaning that for the bulk of her children there was no need to change the rules. And for Anne... Well, when Anne married she didn't see any need for her husband or children to have any titles. For all we know they could have discussed creating Mark and the Phillips children's royals but decided against it.
This was also in the 70s, when the issue of equal primogeniture just seemed to be beginning. Since then, what need has there been to change things? Now that equal primogeniture is becoming a big thing in European realms, Britain is still in a position where any changes would be proactive instead of reactionary - in contrast to everyone else, whose changes are in a reaction to young, female royals in the direct line.
Ask yourself, who would be affected by a change? Princess Margaret's children, who are in (or almost in) their 50s and Princess Anne's children, who are in their 30s. There are no other living female line grandchildren of a monarch, and these 4 have already established themselves as non-royals and aren't likely to appreciate suddenly being royals and having much more expected of them (not to mention more of a loss of privacy).
This issue will be addressed, but not until it actually needs to be - likely not until William's reign, if he has daughters.