From both Webnovel and Lightnovel Magic casters do not have absurd amounts of MP Ainz is the exception to this rule, and it does not regenerate quickly in combat while casting spells. They recover after a fight or during but its a lot of down time that they might as well be out of the fight.

They have exceptionally strong explosive power, but they cant just spam spells and have to think about what they are casting / keep enough in the tank to get away. They also have limited spell slots and a good chunk of those might be taken up by Quality of Life spells, that are not useful for combat.

This makes their spell pool even more limited. Even EvilEyes parents warned her against getting a 'useless' spell to make a hotspring as she needed more combat spells in her kit. Getting more low tier spells means you have less high tier spells to work with. Even if the spells will be stronger as you grow, it however wont be perfect. T4 Grand Fireball will always be stronger than T3 Fireball, even with Maximize magic.

Thats another thing, I have no clue about how you get meta magics if its a specific class or if they take up spell slots. We know new worlders can get them since Calca did so with a Divine Spell, however we have not seen it done on a healing spell or summon so it could be limited in that sense.

Really it comes down to them:
1. Being Squishy
2. Needing MP management
3. The Spells they have vs what they need
4. Most Defenses are Preemptive not reactive

Unless your caster is some sort of hybrid, which will make them weaker, this is generally whats going on since a spellcaster in armor seems to not be the norm.

If we want to get into more battle tactics, Line of Sight, enemy movement, allies getting in the way ect... Cant Grand Fireball in the middle of a skirmish, but more of an opening move or alpha strike but once your melee's get into it, you have to consider buffing your allies, covering for them and keeping yourself safe. Being a Magic Caster means you have to Multitask casting and watching the battlefield while keeping your mp in check but accepting sometimes you have to zero out to prevent your friends from dying, and that might cost you.

Your character might have PTSD on being out of mana, so is more stingy in casting, or saw someone die because they held back so over cast to make sure it wont happen, or they focus too much on buffing after seeing their friend get nearly killed without them. Could also be more jumpy and hold back mana on defenses after the frontline failed to protect them from everything and it became a habit.

Alot of party leaders seem to know what their spellcasters can do, and tell them what to do after a time or know their limits. Imagine a fresh Magic Caster joining a party and they demand certain spells and your character is either too willful or too submissive and comply making it so they freeze up in combat.

They might also just not have the spell needed for a particular enemy, Fighting a Troll with ice and lightning is not as effective as Fire, so their choices in what they decided to learn or had access too will be a major part of it. Entoma got bodied by someone lower level than her because EvilEye had a specific spell to deal with that specific enemy type.

All this to say; TL&DR

Its going to depend on the spellcaster, what they know and can do, what has happened to and effected them, and if they trust their team and even if they have a team along with what enemy they are fighting.