Just like 98% of all front doors are easily opened with brute force. Just because you can circumvent something, or doesn’t make it unusefull.
But the context here is hashing all images in order to catch illicit images, particularly child pornography.
Presumably people who distribute such images take a rather more practical approach to security.
So implementing a universal change to the internet needs to work against those using bank-vault level security rather than easily-picked, easily-booted down, easily bypassed "domestic front door" level of security.
If you implement something that will work in 99% of cases in the net then you'll let through billions of images from those who take a combatative approach to law enforcement. The result being no effect against the criminals you want to catch.
(Your local version of GCHQ will be able to reduce their database size though.)