Because lilies and Thin Air don't *restore* MP. They make a GCD cost zero MP. That's not the same as adding 2400 MP to your bar if you cast Raise under it. Here's an idealized training dummy scenario. I'll be as kind to the dummy WHM as possible.
At base spell speed, you have a 2.5 second GCD. That's 24 GCDs per minute, if you stay on top of your casts. Since Presence of Mind adds an extra GCD and comes up every other minute, let's call that 24.5 GCDs. The cheapest spell you can cast during that GCD is Dia or Glare, both at 400 MP. If you cast healing spells or holy, that amount goes up. So let's stick with the ideal situation, where all of those GCDs cost 400 MP.
400 x 24.5 = 9800 MP down the drain per minute.
But that's just the expenditure. Now to deduct the recovery tools.
Base piety is 200 MP per tick, with 20 server ticks per minute - 4000
Lucid is 3500 MP, unless they changed it - 3500
Assize is 500 MP per use, usable 1.33 times per minute - 666
1634
With those in mind, we're still at a deficit of 1634 MP per minute. Now, you can nullify one of those casts with Thin Air. In that case, it doesn't matter whether it was Glare or swapped out with Raise, Medica 2, or Repose. The cost is zero, and you save 400 MP in that GCD. That gets you down to 1234. You also get two lilies per minute, and you can blow them for their DPS loss but also not spend 400 MP for each of them. That gets you down to 434 MP lost per minute.
To reiterate, that's the minimum MP you can expend in a minute.
Things that will mitigate that: More piety. Dropped casts.
Things that will exacerbate that: More spellspeed. Casting literally anything other than Glare without Thin Airing it.
WHM is running at a gross MP loss just by using their base kit. Any extra spellspeed or healing above three GCDs per minute will make it drain even more rapidly. The amount of piety you need to make up a few hundred MP per minute is (currently) in the thousands.
Once again, WHM is a horribly designed job that seems almost purposefully designed to punish you for playing well.



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