Even Andreessen is unlikely to be structured such that spending millions of dollars at a time on things that don't make returns is a sensible thing to do.
You have to understand the way capital inside venture funds is usually allocated - it's not just one big checking account. A common way to describe the economics of a venture fund is "2 and 20" which means that the fund takes 2% management fees per year (across a 10 year fund lifetime) and takes 20% of the returns or upside.
So, in a hypothetical $300m fund with this arrangement, 80% of the money is reserved for making investments that provide returns. Every time an investment is made, a capital call takes place where the LPs are asked for the money.
The other 20% of the fund goes to management fees - 2% per year although it's not uncommon that these fees be non-linearly distributed, IE 3% in the early years and 1% in the out years when (presumably) new funds are also throwing off management fees.
So your $300m fund really generates $500k / month in management fees. (Probably paid annually or quarterly.) Enough for some very comfortable salaries, ample support staff, great office space and a travel budget but not enough to really start throwing dollars around millions at a time.
Now of course Andreessen has a reported ~$4B under management but I'd be surprised if they're really generating north of $6M in management fees per month. As fund size grows, management fees may come down (since it doesn't take 10x the effort to write checks 10x as large) or a fund as confident as Andreessen (and in as strong a position) might make a "1.5 and 30" deal where they take fewer dollars as management fees up front but get a larger percentage of the upside.
Note, I know absolutely nothing about what's really happening inside Andreessen, this is a purely hypothetical story against what is considered "typical" in the industry. (In the US.)