Because 1) regardless if the filter was engaged or not, it’s another switch in the signal path, 2) if you’re gong to implement it with any transparency in mind, it’s going to require a high quality capacitor (expensive), adding cost to the product, and 3) may require a change (more complexity) the the circuit design.
Why don't all phono preamps include a rumble filter?
In my system, the use of a rumble filter is mandatory (I notice no degradation of sonics when I engage the rumble filter). I'm thinking of upgrading my phono preamp and would like to choose one that has this as an optional feature. However, it appears that most manufacturers of phono preamps do not include this. Why not? Does the use of a rumble filter degrade the sound of a phono preamp?