Stone Chip paint is great on sills, but useless on plastic.
It's there to give some extra protection to the metal and a bit of elasticity to "bounce" stones away. This protects metal panels such as sills, but it won't really stop the colour or clear coat getting chipped.
OEM paint has had weeks/months to cure before it gets to you so has a pretty solid finish. A refinished panel will always be more prone to chipping as it isn't fully cured by the time you start to drive the vehicle. The more professional the job, the better and more resilient the paint (or lacquer to be precise) will be. Therefore rattle-cans chip easily, fully cured professional job less easy but still not great in the first few weeks.