They don't last as long as they say, simple as that.
Me and the Mrs were offered G-3 Glasscoat protection on both our Twingos which is meant to last 5 years, not even just 3!!!
I declined so they said they'd give us the kit to apply ourselves for free. Well they applied it to the Mrs car anyway so I thought we may as well see how it lasts. When it arrived it looks nice and shiny, like you'd expect and it was beading like mad, as much as any wax I've used.
I washed the car with Dodo Juice Born to be Mild so I knew this wouldn't strip the product. Yet after 2 months the beading was virtually non existent.
Now, as I have the kit included with mine I have read the instructions and they are extremely specific. The car must be clean and free from defects. That to me means fully corrected, clayed and de-greased.
Now what dealership applying a product like this will go to those lengths? They will simply wash it and apply the product.
Now these sealants may very well last the claimed time but under very specific conditions. I don't think those conditions are ever adhered too so they simply do not last the claimed time.
Not so much false advertising but very cleverly written small print.
Personally I don't think you can go wrong with a regular detailing plan. Clay every 6-12 months, polish every 3-6 months and wax every 1-3 months.