1. Some people are happy with creating more positive than negative. 2. Some people want to do no negative.
The practical problem with what you proposed is that now I need to vet the charities I donate and make sure its not a scam. Its a hard task to (i) spend 8 hours a day doing net negative (ii) spend more time per day to make sure you as a whole are net positive (iii) be social and have family etc.
From their website: "GiveWell is a nonprofit dedicated to finding outstanding giving opportunities and publishing the full details of our analysis to help donors decide where to give."