I mean I can understand that, but there isn't anything stopping them licensing a few third party titles like Sony, Nintendo and Sega did on their mini systems. The game lineup would be far more interesting with greater variety.
I would have thought that would go without saying.
The point is that SNK license out their stuff to whoever is interested and throw in a bunch of games that they have the rights to. This is the third or fourth device or software collection of this kind that they've done with this kind of approach.
A publisher like Hamster or someone like Nintendo, Sony and Sega might be more interested in getting a wider range of games included on their collection or hardware device.
According to the website this device appears to have been designed and marketed by a company called Unico, so they're working with SNK to license this and have access to SNK's games. I'm sure they could work with whoever owns the rights to some of those other games but that would involve more cost and effort from their perspective.
On a side note, SNK should have the rights to Neo Turf Masters since it was a Nazca game, but for whatever reason they don't include that one.