More or less the castin' craft stuff is just low quality. I've not used it a lot, just some, and the cured casts are not on par with higher quality casting materials. Generally when something's cheaper, the quality is cheaper as the end result.
As far as resin, it is more or less liquid plastic... Resins aren't plastics, they're not the same materials, dont' share the same properties, etc... The plastic Hasbro uses are injection molding plastics that start as little pellets that are heated, and shot into steel molds. Different bird, different chemical properties and things... Just different.
But resins are very similar to plastics. Sometimes referred to as plastics but they are different in some ways too... Easier to work with in small doses is one of their major differences... I have had some experience working in plastics based industry and friends in the broader industry as well as in toys, and there are differences and such.
It should be noted too, resin is a harmful chemical... make no mistake. I know people who've handled and worked with resin all their lives. It's not something to just tink with lightly. Use safety stuff. I suggest working with surgical gloves when you work with it... When sanding resin items, they create REALLY fine dust particles that are incredibly harmful too... Generally it's people working in really large doses of this stuff over years and stuff, but still be cautious. I've heard of resins being the cause of long-term health issues because of mishandling, routine skin contact, etc...