Because the PCs are facing tougher organizations. If a group of 15th level PCs wanted to attack a large city, they could mow through the entirety of the city watch, the militia, 90% of the lord's soldiers/knights, 90% of the clergy, and 90% of the wizard guild without breaking a sweat. Of that remaining 10% of the elites, 90% would be a gentle workout. That last fraction of a fraction of a fraction of a fraction would be a reasonable match for the 15th level party.
For most PC-race-centric adventures, you're only dealing with that ultra-elite portion. It may be because they're plotting evil away from prying eyes, because they're secreting themselves in an otherwise good town, because the PCs get by the lesser guards with a swagger and grin that's not worth mentioning, or some other reason.
I haven't read the Sea Reavers, yet, but if they truly are just shlubs statted as 15th level foes, it's bad adventure design. On the other hand, if there's something special about them, even if it's just that they're essentially adventurers with a pirate motif, then I'm perfectly fine with one ultra-elite pirate ship out there.