Because in 5e NPCs are built along the lines of PCs.
There is no way in Basic, at least that I've noticed, that lets a fighter get WIS or CHA save proficiency, which is the mechanical realisation of "strong will" in Next.
But in TSR editions, fighters at high levels had the best saves in the game (except for clerics vs poison/death).
The structure of fighter saves in 5e in no way resembles TSR editions. The only previous edition that it resembles is 3E.
[MENTION=11821]Obryn[/MENTION] and I aren't expressing concerns because we're unfamiliar with TSR editions. We're expressing concern at departure from the TSR model in favour of (what we see as) the flawed 3E model for saving throw scaling.
Spell casting DCs will typically start at 13 +/- 1 (8 + 2 for proficiency +2-4 for 14-18 modifier). A non-proficient save with an average stat (-1 to +1) requires a roll of 13 +/- 2 for success (30 - 50% chance of success). A proficient character is likely to have 40 - 70% chance of success. Proficiency, gives +10% chance of success and there is a somewhat better chance of having a better modifier because the way proficiencies are assigned (though not always -- Wizards may still have an average Wisdom).
At 17th level, DCs look like they will typically be 19 ( 8 + 6 proficiency + 5 modifier). So a non-proficient save with an average stat will require a 19 +/- 1 for success. ( 5 - 15% chance of success). A proficient character gains +30% and may have a higher modifier further increasing success chances, but that still leaves the proficient character with less than a 50% chance of success. Even if we assume a character will only be required to make saves versus a stat of 20, the non-proficient character has a 35% chance of success and the proficient character has a 65% chance of success.
It does look pretty flawed at first glance. Magic slowly takes over the game as character levels rise as opposed to pre-3e and even how 3e played out at my table.
Content designers need to adjust their expectation to characters will fail their saves especially at higher level which paradoxically means higher level magic needs to be less frightening.