Here's the thing with cancer (1) sometimes it actually does lack oxygen, because it grows faster than blood vessels, so may prefer lactate creation, known as "fermentation" (yes, like beer which ferments sugar to alcohol, but humans ferment sugar into lactate). So the whole body may be bedridden, not exercising, but a little part of it is not getting oxygen. (2) EVEN IF there is oxygen available to the cancer cell, they have been shown to, EVEN THOUGH there is oxygen there, they still proceed through fermentation and produce lactate. And it's not well known WHY cancer cells do this. This paper suggests that it's a chemo resistance mechanism. Other thoughts have been that it simply pushes through more physical carbon atoms which help make more cancer faster.... this concept of "why does the cancer ferment even though oxygen is readily available" is known as "The Warburg Hypothesis"

Because with oxygen 30x more ATP is produced. Why would cancer seek the vastly more inefficient energy production pathway, the insinct is that cancer would be hyperefficient at being canerous.

Dunning-Kruger energy here...