Why do some people seem to disregard the choice of doing nothing (The Zero Choice)?
More generally, you are asking about a common tendency to substitute a sub-set of  alternative actions for the entire set without realizing it.

Beyond the contribution of pure habit, it seems to be related to several common decision making biases, including attentional bias and focusing effect, though it doesn't exactly match any.  I don't recall seeing this investigated directly.

"Zero choice" is a poor choice of terminology, it evokes the "null hypothesis" (as Ben Brocka alluded to in the comments), which in decision making usually means "do nothing", not a call to consider a broader set of alternatives.