It's all down to the silvery tongue and the smoke screen. Some guy turning up to look at a job will know straight away whether the client has any clue about the work needed and the costings, they will over estimate in materials and labour because that is what they have always done. Sometimes it goes well , other times they can take the preverbial and not turn up or leave a kid doing the work whilst they go to another job or sometimes they bail because it's not cost effective anymore, i.e. the money has tun out, They then rely on the client doing nothing and getting fed up of being ignored.
The only way to not let this happen is at the very first contact, the client should ask for references, a written contract and stages payments.
The builders get away with it because they know there is no paper trail to back up any claim