GP here. I started my career in the mid-late 2000s. For me some of the reasons that I remember are:
1. Just plain old not knowing better.
2. Being in NYC, my social circle had a lot of Wall Street finance types. In fact, this is still true - I know more finance people amongst my friends than fellow SWEs.
3. Hacker tech culture wasn't as widespread or widely known back then, especially in NYC. Or maybe it was just me. Going back to #1, I thought things couldn't get any better than dressing up in business casual monkey suits every morning and being ordered to dance like a monkey by a hotshot trader in hopes of picking up after his glorious scraps.
4. I thought the money wasn't half bad, making like $80-90k back then. I thought no one could pay better than the Masters of the Universe on Wall Street.
I don't know what a company like Google was paying their engineers back then, but I think it's a somewhat recent phenomenon where tech SWE compensation grossly and utterly outpaced tech-in-finance SWE compensation. Again, I'm not referring to ultra-elite finance firms like Jane Street or Citadel where I understand SWEs do get paid on par (or better) than FAANG.
I distinctly recall even ignoring a Google recruiter's solicitations back then (2007? 2008?) because I naively thought working at my investment bank was utterly superior to anything Google could offer.
Now I know better, and knowing is half the battle.