Trading systems are complex distributed systems. They need extremely high reliability and low latency, on enormous transaction volume.

There are lots of complex failure modes in a system like this, and minimizing them is an advanced skill. The learning curve for a competent generalist senior engineer to get good at these kinds of problems is significant. Designing and implementing a national securities exchange is a big deal.

This is a place where you want either experienced developers who have worked on other stock exchanges, or at least FAANG engineers who've worked on systems with analogous challenges. This is not a place you want to skimp on salary.