That's because most physics and chemistry experiments don't breed and multiply.
Neither do infertile mosquitoes; your point?
They are talking about something that happens literally in their own backyard.
Really, you think there's no products of modern chemistry in your backyard?
They are right to do a risk assessment.
And there have been risk assessments done, by regulators, taking into account the scientific data. Risk assessments are not something for Joe Bloe and his GED who reads NaturalNews and thinks that "GMO mosquitoes" means that they're going to bite his children and spread a zombie plague.
Changing the balance in an ecosystem can have huge consequences.
Contrary to popular belief, changing the bottom of a food chain rarely has major consequences; it's the changing of the top of a food chain that tends to have the biggest consequences. The higher up the food chain you go, not only do you have more of a profound impact on the landscape (look at how radically, say, deer overpopulation transforms a whole ecosystem), but also the more species tend to be generalists rather than specialists. Generalists means the ability to switch more readily between food sources, meaning changes further down have little impact on them. But if you eliminate a top predator from an area, the consequences further down can be profound.