A number of startups really start having this issue once they start seeing some traction.
Maybe they're actively looking for new or additional senior technical leadership or just scaling the team.
Either way the opportunity is certainly there for companies to hire temporary technical leadership.
The problem here is once they have traction and funding, they likely have a full-time CTO. Fractional CTOs are generally needed at the seed stage when the technical solution hasn't really come together and they don't have this skill-set in house. They also don't have money. It's a slippery slope, and you're better off being a technical advisor (low effort, low equity, low risk) than working underpaid and overworked.