I'm at a very early stage startup and am preparing to make my first hire in the next couple months. To me, it would be ideal if I could consider people who aren

I'm at a very early stage startup and am preparing to make my first hire in the next couple months. To me, it would be ideal if I could consider people who aren't US citizens or permanent residents. But, given how often I see small-mid sized startups saying, "sorry but we can only hire US citizens/green card holders", it seems there are a variety of reasons (that I don't fully understand) that this may not be feasible. Could you share some thoughts on what particular challenges there are in hiring people who do need visas, and if it's possible or reasonable for a very early startup with limited time and resources to overcome those challenges?