It's like Britain or any part of Europe being colonised by the Romans. They left and things fell apart. Who in Europe developed by being left alone?
Nevertheless wherever the Romans stayed longer, so did bits of their way of doing things. We're typing in their alphabet, using many of their words. Our legal systems embed many ideas from the Roman one (I read this but cannot say how true it is). I do know that in my pink bit of Africa our legal system is based on Roman-Dutch law.
We basically believe in educating, building public infrastructure, accountancy, paying tax, a degree of organisation as the Romans seem to have done and we've tried and failed, just as they did, to have democratic government of a sort.
And all of this happened via a 2nd party - the British (and some Dutch).
The problem is that a change in culture and attitudes takes generations and a few generations ago people in my part of the world were wearing animal skins (an exaggeration obviously) but for many the rural life really is only a generation ago.
Now significant numbers of people read and write, work in offices or industry. There's no way that would have happened this fast without modern medicine and agriculture and to a lesser degree the ability to build cities with clean water and sewage. That's because the population would not have been able to grow like it has. Droughts would have wiped people out - without the dams, grain stores and imports-by rail that happened as a result of the creation of these things by people from an alternate culture.
Whats happening now is that because the values of that alternate are only embedded in a small part of the host culture, it's difficult maintain them and therefore things fall apart.
There's a core knowledge/culture about civilisation that didn't come ultimately from the British but really from the Romans and through them from the Greeks and maybe through them from others. It's a multilayered fruit with each previous civilisation adding something. To develop it has probably taken 1000s of years. Nobody has done it totally on their own.