To reply directly to the question... TP is often not preserved because players have a "But, I might need it later" mentality. Why don't they use Elixirs? "I might need it later". Why don't you use the TP Nukes on basic enemies? "I might need it later."
Inevitably, everyone saves their Nukes to use on the bosses... which often renders boss fights... impossible to balance. If you balance for every character to use their TP Nukes on turn one, then you make it a difficult game for those who came into the fight without TP Nukes. But, if you balance around players who didn't bring the TP Nukes, then those who bring them in utterly destroy the fight.
It's really not that big of a mystery why so many games don't "preserve TP". You need only look to the Final Fantasy series and how it's players approach Limit Breaks to see why.
As for myself...
I renamed it "Rage". Oh yes, you dislike this. "It makes little sense!". Actually, no, it makes a lot of sense. As you reach your limits, get desperate, get angry, a myriad of chemicals along with adrenaline dump into your system and give you a hyper focus and a nearly unstoppable amount of power. Most anyone who has ever had a serious fist fight can attest to this. Something in your brain just "clicks" and suddenly your inhibitions drop and the normal limitations your body puts on you also fade away, and the only thing that matters is the complete and utter destruction of the opponent in front of you, even if that means your own destruction alongside 'em.
That's what rage is. It is not, "Oh, I got mad, so I have adrenaline". No, actual rage is the primal part of your brain flicking on and ignoring every other part of your brain in order to do as much damage as possible, even at the expense of your own well-being. It is "fight" taken to its most extreme. If you genuinely see someone in "rage", you fear for your life. Because the logical part of their mind is now gone. It will stay gone until they achieve satisfaction or the chemicals wear off and their brain switches back on. It is legitimate crazy. You can see it in their eyes.
So, yes. I call it "Rage". That is exactly what it is. Characters pulling from their own deepest depths and reserves to become "super human" to some degree.
In most of my characters, there is exactly one skill that uses TP. Just one. That skill, when executed, usually dispatches a single enemy. Or, severely hurts a single enemy. In rare cases, it can wipe four enemies out in one shot.
And then, I have a character who's skillset is "Adrenaline". Which means, every single skill he has (except a self heal) is tied to his TP. Everything he does builds his TP faster, uses a lot of his TP, and does a lot of burst damage (or allows his teammates to do a lot of burst damage).
But, I have equipment in my game with "preserve TP" on it, and even equipment that increases TP gain. Much of it is to allow players to stockpile their TP if they so desire... but they sacrifice other equipment to do so. They sacrifice specific builds to do this.
By and large, my "TP Nukes" are meant to be used against bosses. But, not necessarily immediately. You are meant to accumulate the TP during a boss fight and use it there for spike damage. You aren't meant to grind enemies to build up TP and then go fight the boss.
Still, if a player wants to do that, they can do so. But, doing so makes you "hyper specialized". You'll be good at preserving your TP to drop a Nuke... but you'll sacrifice every other aspect of gameplay about that character in doing so. The player then needs to decide between creating a build that focuses on using a single skill all the time... or one that uses all the other skills and only sometimes uses the single Nuke Skill.
I guess I do have other ways to mitigate the lack of TP as well...
For example, I do have a character that transfers his own TP to either a single target or the entire party. Annnnnddddd I do have items called "Dragon's Blood" that gives you TP (20, 40, 60, 80, 100%) if you use them. So, there are ways to spam your TP, but you have to choose when to do so.
And honestly... I don't think players will. Why?
"Because I might need it later".
You know, in case there's an even bigger giant enemy crab after this first one made of titanium.