They tend to be more mature environments, free of the "brogrammer" culture that's hostile to older people, women, LGBT people, etc. There's not attitude of "let's have a cool, hip office full of cool, hip young people". It's an environment where "culture fit" isn't used as an excuse to discriminate against older people.
It actually turns out that more corporate environments are actually friendlier to marginalized groups than a quirky freewheeling startup.
Job security is great. Big, established juggernauts don't have the kind of churn startups have... there's no worry about "what if the VCs don't go for another round of funding?", and the markets are well-established and slow to change. And if you go into defense contracting or public sector, you might even have lifetime employment.
The work environment is probably going to be nicer. Traditional corporations don't do open offices and don't require engineers to work 60+ hour weeks. Some of us would prefer do to 9-5 in our own cubicle. Banks are also especially generous with PTO (and remember that the "unlimited" PTO you get at startups is a scam)... I'm just going to quote a friend of mine on Facebook when I decided to post a general question of "how much PTO do you get?":
> I used to work for a bank, and they're notorious for giving tons of time, but when in my first position, I had 2 weeks paid vacation, 10 holidays, 10 sick days, and 2 WTFever days. When I was rehired further up the food chain, I got 4 weeks paid vacation, 10 holidays, 10 sick days, and 2 WTFever days, and I could buy an extra week off by lopping a week of pay off my annual salary. If I'd stayed longer, climbed more, I could max out at 8 weeks paid vacation with all the rest of it.
Not all of us care about doing interesting or ground-breaking work. We just want to stay employed so we can fund our lives, and we want a work environment that doesn't make us hate ourselves and want to die.
Honestly, I'm pretty happy at my employer -- we're a tech company, but the environment is very corporate (we're a telecom), and it doesn't feel like a startup at all. The work environment is highly praised, we're ranked as one of the top work environments on Glassdoor, and half of my team are graybeards. I don't want to leave here, but if it ends up happening anyway, I'm giving serious thoughts to pursuing public sector work after this.