Obviously it’s way more complex than this, but even with oxygen, if the glycolysis rate is high, not all pyruvate gets into the mitochondria. U/metacelcus explains it better, but a simpler explanation I use is to think of it like a sink faucet that’s turned up so high that the drain can’t drain it fast, and some water overflows the basin. In this case, glucose to pyruvate is the faucet, the drain is the mitochondria and the overflow is the formation of lactate. It’s not a perfect analogy, but does reasonably well to get the concept across in undergrad exercise physiology classes.