Changing lenses more expensive that buying new frames

I took my mother in for an eye test after an operation at the hospital as she was complaining that the spectacle that they prescribe to her were not fully working they way she wants them to work. I took her in for an eye test with the hope to get new lenses on her frames as they are still new. To my shock, I was told to buy new frames as it’s cheaper than replacing the lenses. How is that though? She has them I just needed new lenses, and unfortunately spec saver refused and they gave me an expensive quote to change lenses only. Also they didn’t even bother wanting to see the frames I brought with. They blantally refused and say it’s cheaper to buy the whole set.