Yeah, but people build hundreds of them.
And... i'm only somewhere between normal and adaptive ai.
If people build hundreds, it's because nobody's properly harassing or pressuring them. Similar to complaints about mass battlecruiser/carrier fleets in Starcraft.
Until I come along with 100 wraiths.
100 Wraiths won't stop a 160-food force of battlecruisers and science vessels, not least because that player will have SCVs and you won't. Wraiths are, unfortunately, too fragile to stand up to a critical mass of battlecruisers, especially when defensive matrix comes into play. Even attacking the science vessels is usually suicidal due to hotkeyed comsats.
If you mix in Lockdown first, however, you stand a decent chance.
A better question would be how the heck you let your opponent get battlecruisers if you have the economy and positioning to get Wraiths that fast.
Goliaths are a decent counter to battlecruisers, though not quite mobile enough.