We can't.

Not because there aren't ways, but because the game systems and encounter design don't support it.

Unless they drastically overhaul that I see no way we can have meaningful character customization (gameplay wise). I've posited numerous times on here about a Materia system where you socketed effects into gear. Each job would have dozens of Materia that impacted various skills in different ways. But the thing is, having all these cool modifiers on abilities is useless if the encounters never utilize them.

Here's a post I made a while back:
Materia should be a gameplay changing system. Yeah I know people will say math this, math that, but you can design it to work, it's just challenging.

Honestly, I'd rather see the devs make jobs have more specific niches, and let Materia be a system to fill gaps in the kit for various content forms, while simultaneously improving chosen aspects.

For PLD:

Here are some examples of how Materia could impact the PLD's Shield Lob skill:
Hit additional targets (if X number are hit do Y effect).
Hit a target past X yalm - allow combo ender/oGCD to be used for free
Increase move speed after hitting target (for on demand burst mobility)
Enemies hit are pulled towards you

Then you assign different colored materia to effects and have different gear come with different number and colors of sockets. For instance maybe throughput effects are considered Red materia so you're limited to how many you can have.

The idea would be that you could socket different materia for different effects and they'd even synergize with each other.

Like you could equip the materia that enables the oGCD/combo ender if further than X yalm, and the one that pulls towards you and boom you have a shield lob that has 2 new effects on that work in tandem.
This is just 4 different mechanics for one specific ability for one specific job. The problem is that if these tools are never needed because situations in combat never arise requiring them then it's useless (and that's the current system we have today).