If you sold that much of it, the price would tank and people would try to find you and kill you.
Satoshi is in theory holding onto ~25 billion in coins. Trying to cash that out would however absolutely tank the current valuations.
But that’s hardly the only major risk. Bulgaria has something like 213,000 bitcoin worth nominally 5 billion or so and no particular interest in bitcoin, but trying to cash that out is again problematic.
I've personally watched sell pressure on Coinbase in the multi-million range get executed over the course of a few minutes, with only a modest temporary effect on the price.
Which is why very large transactions frequently occur either outside of open markets or across a surprisingly long period.