If it were that easy then we could be ass deep in Outsiders, the Gates exist for a purpose, and make it difficult to whistle in the Outsiders.
IMO, Winter (and, in past eras, other defenders of the Gates) keeps the Outsiders from just overrunning Reality. The Gates themselves let Winter go on the offense and help check for Nemesis incursions.
But mortal magic can still summon Outsiders. IIRC it's a free will/invitation thing, like thresholds. The Outsiders can't just rampage in and unmake Reality, but if you invite them in, that's different.
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I think it kind of *is* that easy, if you know something about Outsiders and magic, are crazy enough to do it, and don't get killed by the Wardens.
Which probably actually doesn't describe that many people, so it's not really that easy in practice.
But the bad guys never seem to have that much trouble getting Outsiders...
- Justin summoned HWWBehind before the series.
- The "evil eye cult" summoned HWWBehind in BR... though he didn't actually attack, just threatened Harry, so I'm not sure he was 100% "there" in a physically capable form.
- The Red Court's allies used Outsiders in the big offscreen battles in DB, and also in PG.
- Vitto Malvora is possessed by an Outsider in WN.
- Peabody's mistfiend in TC was probably an Outsider (it's said to be "from the far reaches of the Nevernever", but a later book calls it an Outsider. At the very least, it's Outside-connected due to mordite.)
- There's a
ton of them in the attack on Demonreach in CD, including HWWBefore.
- A pack of cornerhounds shows up in PT.
(I think there's also an Outsider in the graphic novel "War Cry", but I haven't seen that one...)