You're not wrong that the price of ram isn't that much these days. However, this is not simply a case of adding more ram and be done with it. There is a significant amount of extra engineering to be done.

It would in the grand scheme of things be rather easy to "just add ram" but that's not how the product lines work for oscilloscopes. It's a bit like asking "Why does Intel price their high-end chips so high? It's not like the silicon is that much more expensive or yields that differ so much that they're 500% more expensive? And the consumer chips don't even support all the features that their Xeon's do!"

And you're right, it's a digital signal, and in the normal world you would just use a logical analyzer and do it like you said. But this is for educational purposes and seeing the same tool and the actual edges is a great and clear example. That's not to say that oscilloscopes aren't useful for debugging digital signals. After all, seeing the exact glitch might help in some cases.

So yeah, basically it's a combination of product differentiation and not the entirely right tool for the job.