That is because your either doing it for yourself or for a paying customer who appreciates perfection takes time and is willing to pay. Insurance companies don't give a monkey's about a customers car, All that concerns them is getting the customers car back to them for as cheap as possible.I have just finished the restoration of a '73 Vauxhall Firenza bodyshell and I believe the quality of the 'finished job (including paintwork)' is infact better than when new!
They don't care, Most people are not car enthusiast and even the ones who know the repair is bad give up after a couple of attempts of trying to get the insurance company to do a proper repair.As for insurance claims I would expect body repairs to be better than ever ... ...or else they are truly contradicting what they say, claim & preach etc.! - and thus libel for legal prosecution!!!
Very few people are like me and take them to court and make them pay for a proper repair.
I'll actually expand on this and quote you a typical example, A car repaired by an insurance approved repairer failed it's MOT 3 months later as being dangerous due to the way they had (or should I say had not ) welded structural parts of the bodywork on.