You're not missing anything--I remember this question being asked years ago, but we still haven't evolved to the point of being able to deposit bitcoin into your stock brokerage account and buying stocks with it.  I thought Robinhood might have made progress in that area, except that when you buy crypto there, you're not actually in control of the private keys and Robinhood is the custodian of whatever crypto you own with them.  You can't deposit bitcoin, which would obviously be the key to buying stocks with it.

Nevertheless, I haven't been able to understand why anyone would want to do this, since bitcoin in itself is an investment and is usually bought with fiat.  If you've got fiat and a choice of how to spend it, there's no barrier to buying stocks.  My guess is that stock brokers probably don't want to have to deal with accepting crypto from people and not knowing how that bitcoin was obtained (though it shouldn't matter, just like fiat).  A decade later and bitcoin is still associated with illegal business. 

Things might change in the future, OP, but as far as I know converting bitcoin into fiat and then buying stocks with that is the only way I know to go about doing what you want to do.