But plenty of Travellers do get retrospective pp for small chalets packed to the rafters with large families, with no indoor facilities.
In fact, retrospective pp is the only sort of pp most Travellers get. Only about 10% of their initial pp applications are granted. The vast majority of pp granted to Travellers is retrospective.
Most people go into the planning process with a fair assurance that their application will be taken on its merits and will not be the subject of neighbourhood campaigns against them. Most people who go into the planning process know they have a 20% chance of being turned down initially. Travellers otoh know their chances are slim to none, but they still, perversely, insist on needing a place to live.
Given the extent of discussion online of what to do when it is discovered during the course of a home sale that house sellers have quietly gone about home renovation without going near the planning office, it would seem that thumbing one's nose at the law is not something confined to Travellers. Is this cynicism? There seems to be a sturdy and fairly widespread attitude that your home is your castle, and your land is yours to do what you want with, and not just on the part of Travellers.
Appeals to pp decisions are free in many LAs, maybe all afaik. There may be costs involved with document/plan preparation or paying experts to assess or clarify the application.