They are. A proper one goes up and down with the blade. A saw without an in-built provision for that is a surprisingly tough retrofit [0].
A riving knife that doesn't go up and down with the blade is just a pain in the ass that gets removed the first time you aren't making a through cut and rarely gets reinstalled.
[0] https://m.youtube.com/watch?v=4CuchotcM_4 If you've never seen the inside of a Unisaw, there's a lot of parts in that video that weren't there when the saw left the Delta factory. Even if you mass-produced a kit, it would require reworking some of the original parts, possibly at machinist level tolerances.
A riving knife that doesn't go up and down with the blade is just a pain in the ass that gets removed the first time you aren't making a through cut and rarely gets reinstalled.
[0] https://m.youtube.com/watch?v=4CuchotcM_4 If you've never seen the inside of a Unisaw, there's a lot of parts in that video that weren't there when the saw left the Delta factory. Even if you mass-produced a kit, it would require reworking some of the original parts, possibly at machinist level tolerances.