YCombinator funds startups, and startups need scale.
Hiring is very personal, very hard to automate and scale.
Often, tech hiring startups start in a creative way but later become a database of profiles where recruiters can source, similar to Linkedin. Aline Lerner, founder of https://interviewing.io, wrote about this problem here: https://blog.alinelerner.com/ive-been-an-engineer-and-a-recr...
Looking at the link you just shared, she says there's a 45 min recruiter screen, 1 hr technical phone screen, 6 hr engineering onsite, 1 hr recruiter onsite.
That's four different parts that can be outsourced and specialized in. Codility probably replaces the technical phone screen. Onsite doesn't necessarily need to be in-house; both sides dread technical interviews.
I'd rather do a 15 hour interview with a middle man like Triplebyte who can forward my details to multiple startups, rather than do two 6 hour interviews with different companies with a 20% chance of getting accepted. As a hiring startup, we also dread doing those interviews, because we could be filtering out a perfectly good person and it's already hard enough to convince people to join a startup.