Because suicide isn't inevitable. It isn't always some well thought out choice that is carefully made; it is often made impulsively by someone who isn't thinking clearly at all.

If the first thing that pops into your head is suicide by gunshot, and you don't have access to a gun, you can't do that. If the second thing that pops up is considerably more gruesome or scary or just more difficult to attempt than your imaginary gun scenario, you're less likely to even try.

On top of that, suicide by gunshot is the most effective method of suicide by a huge margin. Many more people will live if they try a different method of attempt.

If matters very deeply what method someone uses and what options they have access to. Especially in a country like the US, where mental health care is not as widely available as it should be, and mental illness is deeply stigmatized; and on top of this, the public tends to be very poorly educated on what mental health care looks like and what mental illness even is.