I'm not sure. I'm not a chemist, so I went and looked up a table of bond energies to see whether chlorohydrocarbons should actually be more reactive than the corresponding hydrocarbons, and it seems like the answer is actually yes, because the bond-dissociation energy isn't determined purely in isolation; the rest of the molecule affects it. A Cl-CH₃ bond has an energy of 339 kJ/mol, while the H-CH₃ bond has an energy of 431 kJ/mol, and the outrageously stable F-CH₃ is 452 kJ/mol. In isolation, the C-Cl bond has an energy of some 397 kJ/mol, while C-H is only 337 kJ/mol.
I think typically the issue with reactive molecules is not that they react within the digestive system — if they're reactive enough to do that, then they may burn your mouth or even stomach, but they're unlikely to make it to your intestines without reacting with the rest of your food. The issue is typically that, if they get absorbed into your bloodstream, they can find their way to all parts of your body, and then wreak havoc on the one or a few particular molecules they react with. So apparently sucralose doesn't get absorbed into your bloodstream, and it's not so reactive that you get chemical burns, so it's harmless. And it doesn't get absorbed because it doesn't react with sucrase (or invertase), maybe because the big chlorine atoms bumping around there result in steric hindrance that keeps it from binding to the enzyme properly.
As for insecticides and flame retardants, there is a large family of organochloride insecticides (including DDT), and organohalogens are common flame retardants, including the famous PCBs, but also things like HBCD. Both DDT and PCBs are notable not for being reactive but for being unreactive.