The question of why, this crisis occurred has still not been answered. But, the answer is quite complicated.
First, here's a National Geographic video showing what happened to a village called Houtouwan, on Shengshan Island on the coast of Zhejiang Province. The village, only 49 Km from Shanghai, once with more that 2000 inhabitants, is now nearly deserted with only a handful of people living there. Why?
https://www.youtube.com/watch?v=qr5lbLDowqM
The answer will tell you a lot as to how Evergrande and other developers became so big, All the inhabitants left that village for better jobs in a city.
This has been one of the largest movements of people in history. In 1949, at the last census held by the Nationalist government of the GMD (KMT), it was calculated that China had a population of about 450,000,000, of which only about 90,000,000 lived in cities. In 2020, the urban population had grown to nearly 920,000,000. That's an increase to the urban population of about 830,000,000. Some sort of housing had to built for all those people. Imagine that - to try and place that in some sort of context, it's like replacing all the housing stock of the USA - NOT just once, but 3 times.
If that seems a huge undertaking, consider that in the first 30 years of the PRC, urban housing was mainly focused on replacing the hovels that so many city-dwellers lived prior to 1949, The massive growth of city populations only accelerated after (circa) 1980.
In addition to that, a middle class began growing from roughly the same time, Today, the Chinese middle classes are thought to be around 400 million - larger that the entire population of the USA. As the income of these people increased they also wanted better housing.
The development of high rise apartments really only started in the early 1990's, Australia's former Prime Minister John Howard visited Shanghai in (I think this was the occasion) 1997. In his hotel (opposite the then new developments in Pudong) his staff opened the curtains of his room, to reveal the new highrises (mixed office block's and apartments) and Howard reputedly, was shocked and exclaimed, "Shit!! How long has this been going on?"
In 2001 I made my first visit to Shanghai. A local Chinese friend allowed me the use of a small apartment in a high-rise block in Pudong. And this is my criticism of Chinese housing -the apartment was clean, but rather basic, but what surprised me was the general shabbiness of the whole block. It looked as though they had done no maintenance on the common spaces (public areas) from the time it was built some 7 years prior. I know* that in apartment blocks, you've got to keep the maintenance up or it soon deteriorates. At that time it didn't seemed that anyone cared about maintenance. Maybe its changed, 5 years ago (in Suzhou) buildings seemed to be better cared for.
It was in the first decade of the 21sts C that the building boom really grew in China.
The developers of these blocks are private enterprise, not government-owned companies. And, that's one of the problems. As Sydney (Australia) is finding out. private enterprise companies will often take illegal short-cuts. Here, in Sydney there are a number of high rise apartments, where the builders did not build to regulations and the blocks have developed structural problems.
The problems in China, are not structural (as far as I know) but financial. Developers (it also happens here too) ask for substantial deposits and then don't deliver on time. Management have not kept their eyes on the company's financial position, hence these problems.
Even with this huge building program, urban poor still have problems finding accommodation. This 2019 video by the HK based South China Morning Post, tells their side of the problem.
Where Beijing's 'bottom of society' live - https://www.youtube.com/watch?v=q1QBjpUuflY
To stop Beijing getting too big, the government is building a new model city about 60 km away called Xiong'an, connected to Beijing by highspeed trains
*I Look after maintenance in the block where I live, and once worked in the fit-outs of buildings.