It's not just the skins alone, but also their availability. Having lots of skins can easily not matter much, when they're event-locked you weren't there when they were available. Lack of skins (and their availability) makes it not so tempting to throw much money out on them. On the flipside, if an event happens, that has a paid skin, it makes it all the more tempting to get it, before it's over.
Skins can be a pain, but, with them being pretty much the main source of revenue, at least the gacha is rather generous.