There would little point in having the 'Richness' apply to all the text at the same time, so you need to be able to, for example, when user clicks bold, apply the bold attribute to currently selected test, or if nothing selected, set an attribute that subsequent text follows. But it gets a bit complicated when you consider that there are a number of attributes, when you turn bold on, the italic, highlight, underline etc attributes must not change. So, its not dead easy by any means, not hard but does require some code.
And people like bulleted text, hyper links .....
Every use has a somewhat different set of needs so a ready to use, all coded up toolbar would probably not be very practical, it would 'almost' do 'most' people's job but be quite hard to bridge that final gap.
Qt5 and MacOS does not have a very complete RichMemo by the way, works well on GTK2 and (I believe) Windows.
Davo