I agree it's overused (especially since it's often not actually true) and a particularly tech-savvy grandma would have every right to be offended by it, but at least the "old people are bad at computers" part is is completely normal and nothing to be ashamed of.

It's hard to learn new things, especially if you encounter them late in your life when you don't have nearly as much time, resources and neuroplasticity (is that the right term here? you know what I mean...).

As for why it's usually grandma not grandpa, it's probably a case of old discrimination showing up in today's statistics. If you were denied good STEM education and never got a job that involved bleeding edge technology, of course you were disadvantaged when computers started appearing. It sucks and we should work to never do that again to new generations, but history is sealed and I think making "grandma-approachable" tech is actually a very important goal (when it's actually easy to use, not like this).