Why do my stock dividends have to be reinvested in my Fidelity 401k account? I would like to invest them where I think the best opportunities are.
Dividends from a 401k investment (a tax-deferred vehicle) are themselves considered earnings. Thus, they must remain in the same tax-deferred status. You cannot transfer 401k dividend earnings to a non-qualified plan without paying income tax (ordinary, not the lower qualified dividend rate) and possibly a 10% penalty (if you are under 59 1/2 years of age).
You may be able to do a rollover into a different tax-deferred vehicle such as an IRA or a different employer's 401k, though this is unlikely and would depend heavily on the circumstances and the plan rules.
Depending on your particular 401k plan, you may have a self-directed investment option, in which case you can use those dividends as you wish (including leaving them as cash for a time) but the major point is that the funds must remain in the same account to retain the tax-advantaged status.
By default capital gains and dividends are reinvested at Fidelity. However, they can be changed, and if I recall correctly, you can reinvest one and not the other if you wish.
Since this will direct you to a Fidelity login page, for extra safety google:
"how to not reinvest dividends fidelity" and click on the link that says:
How to Change Dividends and Capital Gains Distributions - Fidelity