I agree. I don't believe we know for sure what causes insulin resistance and that there must be some kind of genetic component to it. However, I also think that lots of people are potentially predisposed towards insulin resistance (as evidenced by rising worldwide rates of T2 diabetes and prediabetes) and that there is also some environmental trigger -- may it be excess carbs or vegetable oils in the diet, chronic stress and sleep deprivation, or something else or any combination of these factors.
There is also an interesting hypothesis centering on the role of fat cells in the development of insulin resistance. If I understand this theory correctly (and I may not) is that there are two types of people.
One set of people develop the maximum number of fat cells before birth or in childhood and these are all the fat cells, they will ever have. Once these fat cells reach their maximum size and are overfilled, they will become insulin resistant and will have problems absorbing any more glucose. Insulin levels rise, but because very little addtional glucose can be absorbed by fat cells, fat now starts depositing on the inner organs, including pancreas and liver (leading to improper functioning, such as lower insulin production and liver insulin resistance and increased release of glucagon from the liver) and eventually blood glucose levels will start to rise.
The other set of people, based on this theory, can grow new fat cells even later in life, so there is theoretically no limit to absorbing excess glucose and turning it into fat. These people would then never become insulin resistant, but would continue gaining weight in response to excess glucose.
What makes this theory attractive, in my mind, is that it can explain why there are some thin T2s and some not so thin T2s -- the difference might just be the number of fat cells we come pre-equipped with.
Interesting theory, anyway, I believe.
If you interested into looking more into this, I think Gabor Erdosi talks some about this and here is also a link to an article which provides some support for this:
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0018284