I don't see what's surprising about it if you account for the context where we're discussing code that operates on large datasets. Aside from banks, do you have any examples? Because all of the examples cited so far do not qualify (Apache, Linux, gcc, etc.), so it seems like most people are completely ignoring that context.
I myself am still maintaining a large system that I started about 20 years ago, and maybe only a couple of functions from that time have survived to this day, but all of that data is still there and valuable.
That part wasn't really obvious, even with context!
Though I'd say that a big reason for that is because very few companies even had a backend 20 years ago compared to today. The ones which did are likely not really consumer oriented, but if you ask medical companies (EMRs), governments, and insurance companies, I'm sure it's not too uncommon to use applications which have codebases that are from before 2002.
Also, 10-20 years from now, I'm sure it will be quite common for companies to have backend services code which are 20 years old. There are still a lot of companies having Java 7 and earlier in production, and that's 11+ years now.
But I agree with what you're saying, it's much easier to get rid of code vs. data, and data is also a lot more valuable!
My former company makes B2B software that's entire purpose is to process business data and our codebase started in 1999 and plenty of original code is still in there. Entire modules have barely been updated, those modules are working and the business needs haven't changed.
Speaking of old business software, PeopleSoft is ubiquitous at big companies and has been around forever. Their codebase probably has code two decades old. The frontend looks like it hasn't been updated in the last decade.
Insurance stuff definitely processes lots of data, and has been around for a lot longer than twenty years.
Additionally, psychometric tests such as the GRE and SAT have been around for a long time and process lots and lots of data (especially relative to when they were first scaled out).
I myself am still maintaining a large system that I started about 20 years ago, and maybe only a couple of functions from that time have survived to this day, but all of that data is still there and valuable.