The following phrases and quotes come to mind.
The path of least resistance means "the easiest way to continue" (here). In your context, you might say something like "People always choose the path of least resistance."
To go out of one's way means to inconvenience oneself in order to do something or to "make an extra effort" (here). In the seat belt example, you might say something like "People won't go out of their way for safety."
You might also consider the following quote from Bob Dylan: "People seldom do what they believe in. They do what is convenient." A paraphrase of the last bit, People do what's convenient, seems like an accurate description of the situation you describe.
You might also consider the phrase convenience is key. It's not a famous proverb as far as I know, but if you google it you'll get many hits linking to discussions of product design (for example, here)—in particular, discussions of the principle that convenience is paramount in getting people to use products.