The odds are orders of magnitude better to win the mega millions than to guess a key with funds at random. You're better off buying $5 lottery tickets than spending $5 on electricity so your computer can make trillions of guesses for private keys. Even if you find a key, relatively few keys have multiple millions of USD worth of BTC. Many many more keys have less than 1 BTC. You're also better off putting that computational energy towards mining a block.