Because they are almost worse than useless. You get the illusion of safety, while not really gaining anything. Ear plugs require proper use to get the full benefit. The Howard Light ones you mentioned have a NRR of 25dB. The average dB of the 30-06 rifle is 163.6 dB. That means that if you are using them exactly as they are designed, you are reducing the dB of that muzzle blast to 138.6dB, which is just barely below the instantaneous hearing damage threshold of 140dB. If you are even just a bit off with how you are using them, which I will guarantee most people are, especially when trying to shove them in while trying to get ready for a shot, you will not get the protection you think you are. Even at 90% effectiveness, using that same example above, you are now only getting 22.5dB of reduction, reducing that muzzle blast of the same 30-06 to 141.1. Over the threshold.
That is why ear plugs are a crap solution to field shooting when using rifles. For shotguns, whose muzzle blasts are generally in the 150dB range, they make more sense.