Because all the effort involved in everything you just described.

Adding paprika to ready salted crisps is pretty effective. I bet it would work with other spices too. The thing is that you can buy one big bag, and then tune the flavours, for example if one person doesn’t like paprika.

If I wanted to put that much effort I to eating something, I wouldn't be eating crisps

Which spices to use and in what ratio is proprietary and I have not been able to create that flavour mix on my own.