If you're a senior C++ lead dev working in the front office of an Investment Bank you can make 250k-500k/year.
If you join a startup as an early employee and get 1% equity , assuming a 10% chance of exit after three years, the company would need to have a $1 billion dollar exit for the risk-reward equation to break-even.
There are a lot of ex-finance developers in the startup world, but overwhelming they're there as founders, as that's the only way they'll get enough equity for it to be worthwhile.
Sure there are other factors at play such as lifestyle, corporate culture, etc. But when it comes down to hard money startups can't compete with investment banks.
1.) IT Admin stuff (Creating accounts, internal admin tools)
2.) Backoffice Dev (plumbing code to book trades, query databases, feed data to risk and PNL reporting tools)
3.) Infrastructure (writing fast code that talks to exchanges to trade autonomously, or writing tools to help people run lots of regressions, experiments, and simulations with ease)
4.) Desk Quant / Quant Dev (writing valuation models, trader UI, tools for trading, tech support for traders, structuring new products, building the grand unified system that processes financial instruments)
5.) Prop Trading (finding new ways to trade for profit; systematizing that into software)
I'd say anyone who is skilled at either 3,4,5 is going to generate more value to the Wall Street firms than what most startups budget for hacker employees.
Working on Wall Street--or most large companies for that matter--is largely transactional. You provide a service. You get paid for that service. You tend to get paid quite well to make up for the abuse you often end up taking. This is particularly true if you work in financial markets (as most Wall Street programmers do) where there are two kinds of people: traders and people who don't matter (being everyone else).
With a startup--particularly an early stage startup--you're taking a long term stake in the company, possibly a significant stake. If it's particularly early that stake but will largely be in lieu of a market wage (or any wage at all).
In my experience the profiles of the people in both categories (in an ideal sense from the company's point of view) don't quite match.
In the case of these grads, it's not an issue of profile but simple arithmetic that results in Wall St winning.