Wouldn't that be safer and perhaps even faster, as humans would arrive at Mars with a city already built by androids?

Humans will not go to Mars and live in cities built by androids. All of that is fantasy and hype.

If we had the technology required to colonize Mars, or even the moon, we could fix the problems we've created on this planet. We could colonize the oceans. We cant't "terraform" anything, we don't have androids that can build anything, we don't have a plan for getting lots of people to Mars, or keeping them alive. That's all fantasy and self-serving stock pumping.

Mars colonization is still a distant goal. With so much of the world unexplored, it's hard to imagine humans living on Mars in our lifetimes. But it makes sense to use robots to pave the way. Japan is already using robots to help with the Fukushima nuclear disaster. Who knows, maybe one day we'll see androids on Mars.

Because Androids don't exist?

> as humans would arrive at Mars

Why would humans go there in person?

Why use androids at all? Bipedal robots are accident prone and difficult to repair. A wealth of effort would be wasted housing and maintaining a transient race of robo-people that would one day be obsoleted to make room for humans. Even then, it's doubtful how effective the anthropoidal form is on a body of mass with 38% of Earth's gravity. You might prefer other options.

When you weigh the risks involved with sending robots that have a limited motor manipulation, range of motion and independent ingenuity, you might as well just spend a few bajillion dollars sending an astronaut do it. You can very feasibly sustain a small population of people on life support, and if they were aided by additive manufacturing tools that could leverage Mars' natural resources then I'd consider that a much more realistic colony moonshot.