Because the increase in a share's value is tied to a company's profits and losses, while the increase in a cryptocurrency's value is tied to hot air and memes (exhibit A: Dogecoin).

Because an index tracker exposes you to the risk of falling indices. Using a solid stable coin (DAI) and earning yield using an established lending platform (Compound or AAVE) or liquidity pool (Curve) does not. However, these new financial tools of course have other inherent risks but they are probably smaller than exposing oneself to the wildly fluctuating Bitcoin price.

> a solid stable coin … established lending platform

I think we are probably eons apart on the meanings of the words “solid”, “stable”, and “established” here.