Michael Jackson became whiter because he suffered from a disease called Vitiligo. He also had Lupus which was confirmed during his autopsy. This condition causes a loss of pigmentation which can spread all over the body. Your immune system destroys your pigment cells. It was noticed on his hand, which is why he started wearing the one glove in order to hide it.

https://www.mayoclinic.org/diseases-conditions/vitiligo/symptoms-causes/syc-20355912

It then started spreading quickly, which caused him to use skin cream and makeup in order to have a uniform appearance. Can you imagine Michael appearing in public looking like this?:

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His career would have been over. At first he covered up the small spots with dark makeup, then the spots were so extensive, that it made sense to use skin whitener to even the skin tone out.

You can see a little bit of it here, underneath the makeup:

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https://www.truemichaeljackson.com/health/vitiligo/

It starts to make sense now that he was often seen in public with a mask as it affects the mucus membrane and mouth, and an assistant carrying an umbrella, as they are at increased risk of sunburn and skin cancer due to the missing pigment.