Why didn't my Dentist pick-up on tooth decay?

My 5 year old had to go back after his regular dental check for a deep filling. He became too distressed whilst there, so the dentist referred him to Guys Hospital. While we were there the consultant told us that not only would that tooth need removing but 3 others also could not be saved & would have to extracted, and four top teeth will need metal caps. We brush his teeth regularly, he rarely drinks sugary drinks, never drank from a baby bottle but does have occasional sweets ( I think most kids do??) I feel dreadful that his teeth are this poor, and obviously we should have been even more careful with his diet but he has been having dental check-ups every 3 months since he has had teeth, surely the point of this is to pick up problems early & treat them? My poor boy now has to undergo a general anaesthetic & multiple extractions & I feel I followed advice given :-(