Insulin is an hormone. Any anormal hormonal level is bound to have consequences. Here it's called hyperinsulinemia if you want to read about it.

https://en.m.wikipedia.org/wiki/Hyperinsulinemia

Insulin is a key that open the cells doors for sugar to enter and be used as fuel.

In insulin resistance the keyhole is blocked.

The body thinks there is a lack of keys because cells are screaming for fuel and it knows there is fuel. So it makes more insulin. This works for a time, basically forcing the locks.

But if not controlled even the excess insulin is not enough anymore.

The body is flooded with sugar it can't use and it ends up with diabetes.

As to why the keyholes get blocked... why does anything else go wrong in our bodies? Sometimes it's genetics, sometimes it's environmental, sometimes it's a combination of factors (like the change in hormones and the weight gain from pcos).