Because without the coded final approach fix, the navigation box won't transition into approach mode. This reduces scaling, and alarm limits that allows you to use the navigation system in the final segment. This is also why you require RAIM.

ICAO standard final segment length is 5nm.

yes I’m aware of that but there is no reason that the FMS can’t do that without having to have the extra waypoint there, it should just happen at 5NM

There's plenty of reasons why the FMS/Nav can't function as you described.

Wouldn't that just be a software thing? Recognising you're 5nm to the point and start to transition?

Which 30 + year old GA aircraft have an FMS?

We had this discussion some years ago and Ozexpat (I think it was) made the same statement as Alpha above.

At the time, I pointed out that the GNS-XLS box (not really an FMS) we had in the 146 scaled in to +/-0.3nm at 2nm before Foxtrot, so it seemed to me that the box could scale without having to cross a waypoint. Certainly, the fact that Foxtrot exists is most unfortunate and really cruels what would have been a terrific safety "upgrade" compared to NDB and VOR approaches. I am sure that if they were designed today, they would not be done that way.

It's called a "pilot" ;)