Because they don't improve the odds if a person is careful about reloading. What they do for those people is give peace of mind.
They are a convenience and should be factored in on that account, not because "they improve safety." As I have said before a non careful person can load bad ammo just as efficiently on a single stage as a progressive. They can load bad ammo just as carefully on a machine with no auto index as they can on a machine with auto index.
IMO, and only my opinion, manual index allows me to check things easier. After weighing my shooting needs the first press I ever bought and reloaded on was a Dillon RL550B. I had no mentor. I still have that press and have loaded thousands upon thousands of rounds. Never a double charge, never a squib, never a overly long rd, etc. Why? Because I have the ability to read the manual, learn a machine, and watch quality control.
While I do believe a powder check die decreases the chances of a squib or double charge what if the battery is dead? I do not believe the auto index adds anything to safety.
All that being said I will say that both the Hornady and the Dillon will work very well for the OP. Just weigh your skill, needs, preferences and buy the machine that you believe will work best for you.