If you generated weak keys then it's not theft for me to guess/calculate them and allocate those coins to myself.
This is why BTC is described as proving the strength of hash functions, etc. If it was easy to cheat, someone would claim the public bug bounty by giving themselves all the coins. Because they haven't, we have a fairly good minimum bound on the difficulty.
But giving yourself all the coins means demonstrates that the system doesn't work conclusively, and guarantees that no one will accept the coins for goods and services, so while giving yourself "all the coins" destroys a lot of value, it doesn't actually capture any value. So, if you wanted to profit from an exploit you developed, you would not do that (if you wanted to discredit bitcoin and had developed the exploit, you would do that.)