One possible issue with a casual no with some exceedingly persistent askers or bullies who won’t let up is that they’ll ignore the no behind it and regard it as a sign they need to be more persistent/aggressive in asking.
With such folks, sometimes a firm no explicitly expressed is the only way to unambiguously put such askers/bullies and other prospective askers/bullies on notice that they’d best be moving on to those who are more generous or easier marks.