I don’t care whether people say hello or not, it doesn’t bother me either way unless it’s remote enough to be awkward not to do so.
For me it’s about whether someone might need help, and that’s more likely to be the case on a trail than on a road due to the relative frequency of passing users. There’s also more of a natural opportunity to be friendly if riders are stopped or moving slowly.
There’s another aspect to it when I occasionally ride my road bike. I’m rarely carrying any spare spares to be able to offer much help, which is especially the case if you’re riding a mtb. I’m not going to give you my only co2 cartridge or tube in case I need it. You can have a patch. Really you should have been prepared though and have a phone anyway, so you can wait to be picked up if necessary. I have helped people in other ways though, such as leading them back to somewhere they know, or helping them use tools, or making a call for them.