The last I heard FF Remake won't use it and KH3 dropped it a while back. I mean it looks perfectly fine to me, actually better than fine FFXV looks fantastic. So why is SE not using it in their other games. Do games take longer to make on it?

Probably because Luminous Engine is starting to show its age on the development end of things (I presume), and UE4 in particular is fully featured enough that unless super specific things are needed, it can be used as a pretty all purpose engine for the current generation.

Can't they just tweet it like what they did with the UE4? I mean didn't the first UE4 build release the same time as luminous?

First off, KH3 didn't "drop" anything as it was never said it was using luminous engine to begin with.

Now to answer your question, what I think is that there never actually was something called luminous engine in an engine sense of the word. There was probably a tech demo or two here or there and SE being SE just claimed that they had some sort of super engine up and running.

What I believe is happening now is that whatever engine is being used to make FF15 and is being called luminous isn't really an engine at all. its just a collection of very specific assets with even less documentation that's being built on the fly to suit whatever the team needs for FF15. So adapting the engine to any other group of devs would be extremely difficult.

Because I don't see how it would be possible that any company actually builds an engine over the past 5 years, then start making one game with it and then completely ditches the engine for a third party engine for all other games being made in the same timeline.

Edit: Actually, I think this topic is far too presumptious. It's not necessarily that Luminous is gone (I mistook its age, my bad), but rather, it may just not be complete enough to use exactly as needed. After all, it was only unveiled a few months ago as a presumably complete engine, and even though XV does use it in part (as does XIV), it really is just to add features not available with the Ebony engine they developed for the game.

Edit 2: Hell, it's hard for me to determine just how much of FFXV is being done with Luminous. As far as I can tell, they may have done a full switch with an intent to be up to 2.0 by release.

I could be wrong but it might be a case of value for money , going to UE4 means they don't have to keep up the ongoing software engineering supporting for LE , I'm sure most of us have heard different developers say the cost of creating and maintaining a proprietary engine in this day and age is in most cases unfeasible unless you have a raft of games using it.

The fact is by licensing UE4 they have at their disposal an engine that can be used across a broad set of their games ranging from their Japanese core square-enix titles through to the western eidos titles.

Ikr? FFXV looks awesome. I just think SE really needs to optimize this engine because i'm pretty sure it could help them out in the long run.

Last time they lost focus, it ended up as last gen nightmare. Moving staff around would be a terrible idea so IMO they should keep developping the engine alongside FFXV and not slow down other projects to work on an engine that has an uncertain future, no matter how good looking FFXV is.

Probably because they're not developing it together with FFXV.  So until FFXV is done then they can't really use it much. 

If it speeds up their development times then I don't really care what engine they use.