Part because they're hand made and hand cut, part to recoup the cost of R&D, possibly part because it's a collaboration (I'd think Sephora and Hakuhodo would split profits in some way). The prices are close to, or a little more than, the MUFE brushes (also hand made and hand cut with synthetic bristles, though not a collaborative effort), if you want a like for like comparison.
Shape-wise, I think the concealer brush and Fan Cheek have a unique shape compared to Haku's regular range. The concealer brush is about on par, price-wise, with the concealer brushes they do offer, and the cheek brush is cheaper than some and more expensive than others.
The Small Teardrop Highlighter is somewhat close to the G5521BkSL and is only $3 more ($38 vs $35). The large one has four dupes and is cheaper than all of them ($49 vs $88 for the J103, $97 for the S103, $115 for the SJ103, $75 for the B103BkSL). The Wedge Sloping Powder is sort of unique; Hakuhodo have a few brushes that are similarly shaped, but the bristles are longer. The Wedge Sloping Powder is cheaper than all of Hakuhodo's offerings, though ($54 vs $100+).
Another thing is that Hakuhodo doesn't offer purely synthetic face brushes, apart from their flat foundation brushes. Any synthetic bristles are blended with natural fibers. So that's another selling point for their collab with Sephora.