No, they really don't. I'm not talking about "some" but "most." You can go take a look at real-world performance numbers and for a good NAS, you're looking at over $500 just for the enclosure without any drives at all. And you need at least 4 drives or you won't have any redundancy. And without redundancy, you shouldn't be using such a product unless it's strictly for a scratch disk. At this level you may have something near 90MB/s, but even then it's not guaranteed.

Remember that there are a ton of NAS products out there. The best performance in consumer devices is typically from the bigger players like ReadyNAS, Synology and QNAP. Not WD, Buffalo, Dlink, etc.

And even these guys have plenty of products in their lower price points that don't get anywhere close to 90MB/s real-world read or write.

It's one of the reasons why my ReadyNAS Pro used to cost over $1000 just for the enclosure.