Each line on its own is indeed simple. But you have a few thousands or ten thousands of them in 1 transaction or batch process. And that batch process is just 1 in a long chain of batch processes. So you'll need to get a grip on 10-100K lines before you understand the whole process. Procedures are easily 1000 lines long, and any line in there can undo any other line if someone from 30 years ago did not find another way to fix a bug.
There is no abstraction, so code duplication is everywhere. 5 things that seem mostly identical, except in some edge cases means in reality 1 that is correct, 3 that were forgotten in the update process, and 1 that really is different.
Every company is different. As there is almost no human movement, and this is from before networking was widespread, knowledge from each company is an island on its own.
The thing is deeply legacy. Ideas like encryption, unicode, a hostile internet network, ... are alien to the COBOL world. The underlying assumption is that the central computer is holy, and humans need to adapt to its whims. You'd better change your name if it contains an accent not available in the local EBCDIC variant. Putting an € sign in a string? That newby char only exists for 2 decades, hope someone migrated to a code page containing it. On the pro side, Hacking attempts are only allowed in the business hours, as the thing switches to batch processing mode at night.
An organized testing methodology is rare. The cobol world depends on the knowledge of the programmer to remember the edge cases. No things like automated unit testing, maybe a private db with a few records.
The data model is horrible. Choose the size of a record and live with it. You want more chars? Find some spot that's still free and put it in there. The overlays, C unions on steroids, mean any location can contain anything, and any missing IF means you're overwriting someone else's data. I saw cobol devs argue for truncating email adresses. They'd done it with physical mail adresses too, and the mail still arrives. Those mail servers should learn to guess the missing chars at the end, just like the postal workers did.
You can use COBOL in a modern setting, but it is not COBOL itself that is the problem. It is the whole ecosystem around it.
And of course, they want you to have a 30% pay cut, as COBOL is easy.