Because $. For ambitious 20-something youngcels, there are other routes to a strong career start, of course -- politics, think tanks, and lobbying; civil service and diplomacy; corporate law. Big tech, engineering, and pharma are good too (think Facebook, Amazon, Google, Boeing, Lockheed, etc), but since I'm not a STEM student I don't know too much about this sector. Startups are for people with a huge appetite for risk and a strong financial safety net from their family.
Finance is a huge draw for some people, partly because of the prestige or partly because they're under the (somewhat mistaken) impression that you get to party like whoever the fuck that guy was in The Wolf of Wall Street.
There were a few kids at my boarding school who, at the age of 16, already thought they knew what they wanted to do in life:
1. Ivy League undergrad
2. Investment banking analyst (preferably at GS/MS/JPM/BAML)
3. Harvard/Wharton MBA
4. Investment banking associate, VP
5. Break into private equity, hedge funds (places like Blackstone, KKR, BlackRock)
7. Retire early and buy a home at Martha's Vineyard or someplace similar after making a shit ton of money
8. Politics, maybe?
Life doesn't always pan out that way. I know a couple of kids who wanted to go this route but over the last couple of years, they got bored of finance and became interested in consulting, international relations, law, or journalism.