Since almost every high school senior will have significant cost constraints (perhaps only 1-3% will have parents with enough money that paying list price to any college is no problem), the cost constraints are typically the most important factor in college choice for most high school seniors.
So a safety must be selected from those colleges which are known to be affordable. For most high school seniors, the out-of-state public schools and the less selective private schools with poor FA are unlikely to be affordable without competitive merit scholarships (which would make them non-safeties).
It is entirely possible that some students will have no affordable safeties. For example, consider a student from a lower to lower middle income family in a rural area or “education desert” where there is no low tuition minus FA university within reasonable commuting range that is an admission safety, and who does not have the stats to earn any of the few remaining automatic full ride merit scholarships.