Why has apples equity been going down?

I was looking at the balance sheet of apple in a video I was watching and it showed that apples equity has been going down a lot from 2017 to 2021 .When they looked closer at the equity statements they concluded that it was because they were doing large share buybacks, so cash was used to buy the stock ie assets were reduced as cash is an asset.

However even though cash was used to buyback stocks , the stock in itself is also an asset and the price of apple stock has only gone up from 2017 to 2021 so this should more than counteract the effect of assets reduction due to cash being used to buy back shares and thus equity really shouldn't have changed significantly because of that .