At any software company you have to learn new things. Every company has its internal development stack and custom software, unless the company literally isn't a software company at all or is doing something so unoriginal and dull that you wouldn't want to work there.
I'm all for training and learning, but pretending that anything can be picked up quickly and easily is just wrong, especially in a trading environment.
Any typo or bug is going to cost millions of dollars, it's very hostile to newcomers. They will be struggling enough to learn about finance and the company.
Good job they're using a language and other techniques which ensure that bugs are caught early, often at compile time.
a compiler can’t catch a bug if you calculate interest rates incorrectly, for example.
Building your own frameworks usually saves some time at the beginning, but the costs usually outweigh the benefits later on.
Sorry to be pedantic, but the distinction between libraries and frameworks is important, in my opinion.
It is purely a matter of volume - are you doing enough business to make it worthwhile?
The last company I worked for had in-house database, programming language, IDE for that language, version control system, job scheduler... At the time they started doing that there was nothing available that did what they needed, and even now, their stack does things that I'm not sure I've seen outside, tho' 10 years on it's less clear that it would be worth starting a project like that from scratch now.
Remember, in business, there are no concepts of "cheap" or "expensive", there is only "worth it" or "not".