It's an inevitable consequence of a good hash function. Every string you try is going to give you a different hash, so you just need to try enough strings.

But each time you add another digit to the prefix you're looking for, the difficulty of the match goes up by about 16 times.

If you're only looking for a few digits in the hash, then any brute force that is capable of changing the string every time you attempt the match will always find it. If you're looking for seven digits in the hash only, then it'll take on average 134217727 attempts. Which isn't really all that many on modern hardware.