It boils down to design philosophy, in the sense of where or how you design for a specific goal. There's no real need to put that kind of feature on the pickup itself - people are used to tone shaping happening in a later stage (passive or active controls on the bass, pedals, amp, etc.). Designing something that takes a different approach could raise "compatibility" questions. Or it could limit the flexibility of what you deliver.
Tone shaping in the form of simple filters is pretty easy. It's a few cents in parts that take up a few square mm of space. The problem is, if you hard code that in the pickup, it can't be undone. Whereas if you leave the pickup "pure" in the sense that the tonal characteristics are only a result of the coil/magnet, you're still allowing for simple filters or active tone controls, but now you can sell them separately instead of building them into the pickup. This makes much more sense from a marketing and practicality sense.