Because that is how I would prefer it to be. I see the ADS as easy mode, others see the FSS as easy mode. lets say they are both easy mode, combine the two and it becomes easy easy mode. I don't think thats a good combination for any game. Thats my opinion on it and it won't change.
That's a false logic because the tools don't entirely overlap.
There's plenty of room for a reasonably functional system scanner and a reasonably functional body scanner to exist alongside each other even with some of their results overlapping.
What having multiple tools offers is choice, and it's exponential.
One tool - one method only.
Two tools - at least four methods available based on using one or the other or both in opposite orders.
You just have to look at the vast array of combat options made possible by the huge array of tools on offer.
Yes, there will always be a meta that many gravitate towards, but there are many players who outfit and play it their way because it brings them joy.
The FSS-only option clearly brings you enjoyment.
Don't be so closed off to enabling the enjoyment of others who don't enjoy the FSS being obligatory but were also a bit jaded by the ADS.