Why are women who’ve been with their bf for years so shocked during a proposal?

Like if they’re already sharing a life, living together, having sex, paying bills together, maybe have been together for at least four years, maybe already have a kid together, why are they so shocked being proposed to?? Did they think everything they’re sharing together is temporary until he proposes? Why is waiting years after playing house a good time to propose, why propose you’re already doing the things…?