A couple reasons: math, flexbility, and the inability to self-direct.

Yes, there's a 100% return in the first year, and then what? Bitcoin will put out multiple 100% returns over the same several decades.

You're giving up freedom money for locked up money.

Many 401Ks, including mine, are not self-directed, or at least not completely self-directed, meaning you can't treat it as a brokerage IRA. Your options are limited to basket funds, typically labeled "Target 2050" or some such nonsense.