“Writing code well” includes several relavant things:

• Security implications
• Stability and edge case handling
• Performance
• Quality of output (ie. for UI/UX)
• Readability (Agents need to read to make changes too! Violating DRY/SOLID/etc. could still cause issues!)

As far as I’m aware you do currently still need a human to ensure stuff like this is followed. People use agents because they don’t care about the above, or because they can get close enough and intervene to fix any issues that appear.