If you've got a strong track record and connections then you can raise money with just an idea, but in reality most founders don't have that so the only way they can prove their ability is via their actions and these days that tends to mean at least building something to prove what you're capable of.

BTW. Andy Bechtolsheim invested in Google three years after they'd started working on it (as part of their Phd), they'd already launched a public version (google.stanford.edu) with real users by the time he invested. Justin Kan had been working on Kiko for a year before YC invested. Alexis & Steve had been working on MyMobileMenu for a year before they pitched it to YC and PG convinced them to pivot.

It's easy to romanticizes away the slog of work that goes in before a startup raises money, but many successful founders had years of work before they raised seed funding.