"Why don't they go and recruit the commoners"
Do you go out and hire every random people on the street on the expectation that one of them will be the right one for the job you offer or do you at best put up announcement and wait for someone to come?
Now would you put up that announcement in the homeless area? Probably not, right? Same damn thing. Right now the nobles are already spreading thin their resources they don't have any to spare on the 1/Gods know the chance of someone like Myne (who did die, might I remind you, before Urano came into her body)
Any consuming who survive til baptism without doing magic compression will have so little magic it's not worth recruiting them (and they likely won't die before reaching the 'average' age anyway)
And those who might have enough like Freda? Either dead before baptism or in case of her make a connection already (and in her case, the person contracting her is on the LOW end of mana pool for nobles, the higher ranking ones basically have so much more mana/demand for mana that they wouldn't need it)
Myne's magic power level is a special case because of her magic compression (squeezing and pushing it back into the box) increasing her total mana.
Furthermore, and this apply to the nobles themselves too, in the eye of society you don't count as a person until your baptism.
If you die before your baptism? Maybe your family will be sad, but nothing more (in fact, according to the wiki, there was 2 children born between the parents after Tuuri and before Myne, obviously neither of those survived)
In fact, for the nobles you're not supposed to let your children come into contact with any nobles outside of family until baptism.
"They found a plant that can absorb excess mana!"
It also spawn toronbe, making it also a threat (grown toronbe require a squad of flying knights to handle, meaning you potentially make other people waste a lot of mana just to save maybe 1 person temporarily)
It's also seasonal, so there's limit to how much you can keep around.
Myne herself literally spawned one in that chapter just by holding it for a few seconds, the risk vs reward is not balanced for someone with high mana like her.
(Heck, she almost die at one point from freshly grown toronbe, and only survive because the knights rescued her right away since they were fighting a fully grown toronbe nearby)
"The nobles are hoarding the magic tools!"
No they're not, in fact the people at the temple are usually children of nobles who they don't have the money to make magic tool for, and can only send them to the temple + donate money to the temple for their living expense
Think about it, giving someone a life-time living expense is CHEAPER than preparing the magic tool + magic stones for them.
(they're also limited by how many magic stones of the right quality they can get)
"Why don't they just teach commoners magic"
Because you don't just wave a hand and magic happens, a noble have to go through school to not only learn how to use certain spells, but also go through a few things for it.
1. Getting Blessings from the Gods. You basically go into this room and make a long prayer to EVERY Gods known, and if they like you enough they'll give you their blessing. You have to remember all the names of the Gods and recite them perfectly, otherwise you'll fail (though it's not mentioned what's the consequence of failure it, at the very least you'll definitely miss out on the blessings)
The blessing essentially increases your magic efficiency, with more blessings = better efficiency.
How to get Gods to like you? Offer your mana to the Gods in prayers (usually the mana goes into divine tools that'll be used for some purpose like creating magical fertilizer, so it's not wasted)
2. Learn magic compression - That's right, the thing Myne has been doing since becoming Myne is actually something nobles start doing after getting into school. Why? Well, see previous Myne for what happen when you fail to compress it.
3. Get the Staff - This is a near literal divine tool that give you access to special magics activate with just a keyword. You go through this holy ground where you keep going until you 'notice' a thing that when picked up will be absorbed into you and after a period of assimilating (ranging from Myne's hours to a few days) then you can call on your own Staff. It's a very delicate process and during assimilation period you're supposed to remains isolated to avoid magic influence from other people affecting your Staff.
After doing those 3 steps (the compressions thing is done even in adulthood, since after your 'vessel' stopped growing it's the only way to increase mana capacity), the level of your magic will be far beyond anyone without them can compare (not to mention you have access to spells they latter don't)
To compare the difference in efficiency:
the time it took for Myne to clean up a devastated land - hour(s)
the time estimated by the Priests from Central temple who showed up after Myne was done with that - days, with multiple groups taking turns to channel it throughout the day
ONE person who's gone through getting blessings, the Staff and lots of mana compression > a whole group of priests (these aren't any low ranking priests either, a normal priest at Central temple would be even high rank nobles in some domain)
