Because customization is extremely hard to balance. Sure, if you have one person using a mix/max efficiency build, it's not a big deal right? But what if you have a full party using one? The party is suddenly a whole lot more efficient than party that's not using any optimal builds. Then which one do you balance the encounters based on? If you use the min/max end, you'll get a well balanced fight for them but for everyone else the fight might be impossible. If you go for the other end, you make the game too easy for the people who actually know how to build an efficient customization.

That's the problem with all customization options in every game possible. Either it has to be very, very negligible or cosmetic, and in both cases it's completely irrelevant at that point. Or you could just have a completely unbalanced game, that's fun too.

Now different skill trees to do the same things with different abilities are a valid point. But I think we're getting those in terms of different classes and jobs. Please look forward to it.