I'm not saying they couldn't find it, or didn't, there is the ruins of a settlement on it. However unless you have it on a chart, it isn't going to be some you'd easily run across. The Lakes are very large and can be very dangerous in storms. So if it is removed from charts for whatever reason, not easy to find.
There's several contexts of "finding it" under discussion, and it's worth clarifying what we are talking about.
Mundane folk need charts/maps, or aircraft/etc. So far as we know, the White Council has effectively edited the island off all the charts & maps. The WC may even have gotten things like satellite-overview to apply the same "editing" that military bases get, so it doesn't show that way. It's likely that some local pilots know of it; short-haul passenger & cargo aircraft will occasionally catch sight, no doubt. That's about it.
Wizards mostly seem to get there via Ways (except, for some reason, Harry Dresden(?!), who (
despite owning the Ways of LeFey!!!) keeps
taking a boat there! I find that
very odd indeed (I sense a spin-off thread in the offing)). Wizards -- wardens of the prison -- have been getting to the island, and stashing prisoners, for... I dunno... Centuries? Millennia? The prison on the island is
O-L-D (in part, of course, because it was constructed via Time-based magic).
My point is: wizards, so far as we know, have never had any particular challenge "finding" the island, in the "finding their way there" sense. The only issue for wizards --
maybe -- is
locating it, as in a location on a map.
My point (in this tangent) is -- wizards have
not in fact had any particular challenge in finding the island, in that 2nd "on a mundane map" sense. Only muggles are challenged (and even them, less and less so); but they have no particular desire regarding the island. Wizards, OTOH, have an intense interest in it.