It's because in practice, a MAG build Eirin actually does abominably bad damage in comparison to everyone else. 300% is actually really low. Notice how most others gets 400~500% on their trashclearing skills? Her MAG stat isn't as high as it might seem by her growth either, due to her sluggish level rate.
And no, you can't try to logic me out of this, because this isn't from numbercrunching alone, it's from actual use of her.
She'll be able to stand up to other character's NORMAL trashclearing damage if she's hitting a WEAKNESS, which is a testament to how bleh her damage is. The main appeal of them is supposed to be their offense stat debuffs, but of course, these don't usually land on things you want them to, and since she's so slow she's not exactly the best for a debuff tactic on a boss (Although since she's kinda tanky it'd work okay I guess; probably the best way to use her, albiet still bleh)
She gets better after she can spam Astronomical Entombing, but that's late plus disk, and then she's just a worse version of Yuyuko, who is much better for spamming an expensive SPI nuke.
As well, with her lvlup rate, her defensive stats aren't quite as good as they might look at first glance (Although a defensive build makes her a decent tank, for sure). But, her heal is incredibly slow, and very lackluster because Minoriko can fullheal, Sanae should be able to at least mostly heal most characters, and later in the game even Reimu can nearly fullheal; while Eirin's will never get better.
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Going for HP builds over def/mnd builds on characters is a REALLY bad idea. They'll take far more damage from attacks, and the result will not be anywhere near helpful ever, apart from trying to make Meiling able to take Last Judgment. This game uses subtraction formulas, so on tanks, DEF/MND is the way to go. Even Komachi is much better off going for an offense build, because there's not a lot to gain from doing HP growth on her (And she's actually a decent attacker)
Not only that, but Minoriko should be able to heal far more then Eirin even on HP-build Komachi. So. You'd be way better off not using Eirin even for HP build tanks (which are still a very bad idea), not to mention that Minoriko's heal is far faster and she's much tankier MND-wise, and possesses a useful buff as well, and is very cheap (Comes in great handiness for the couple Djinn Storm using bosses)