Maybe they need someone sufficiently senior to take over parts of the project.
Maybe they can not afford to / are unwilling to compensate for the role - FAANG still out-pays most other companies.
Maybe they've lost a couple of senior hires - finding the right fit is difficult.
I don’t even see Principal Engineer listed as an open position on the link they provided in the AD or on YC’s job section:
https://jobs.lever.co/buildzoom
https://www.ycombinator.com/companies/buildzoom/jobs/
Here’s screenshot of the HN AD as a point of reference, since it will disappear at some point:
https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...
Junior -> Mid-level -> Senior -> Principal -> Staff
The rank in that scheme doesn't imply head of anything, not even head of any project. It may or may not come with a relatively high degree of autonomy. It doesn't imply only one of anything.
If that's how BuildZoom are using it, advertising for a Principal is very similar to if they were permanently advertising for another Senior Engineer. If they end up hiring a lot of Principals who have come from other roles at a simialr level, I don't think there's anything odd about that.
I style myself as a Senior on LinkedIn. The title doesn't mean anything formal because I'm self-employed most of the time, so I choose my own title to convey a reasonable idea of what I can offer. I had formal employment with a Senior title 24 years ago, so I'm pretty sure I can stake a fair claim! It had never occured to me to use the higher titles in the above list, though nowadays as Principal/Staff gets more common I'm wondering if Senior looks too inexperienced now.
In late 2020 I applied for a role as Senior Firmware Engineer, and was instead offered a role as Principal Firmware Engineer. When I asked what that title meant, the company said Principal came with higher RSUs as proportion of the compensation, because Principal Engineers were expected to take more of a lead in defining company projects and mentoring others, so they should share a little more in the stock risks of the company.
Well, who'd complain about more autonomy and pay. However in reality, it appeared to be the same kind of coding work with about the same autonomy as they'd give a Mid-level or Senior, with a title-based reason to pay a bit more and some recognition of experience.
(I accepted the role, but after a long time preparing to start, they ghosted me and the recruiters. This was after we had agreed terms and starting date by email, and told me to expect the new-employee pack by post. Turns out their executive team's secret policy, not mentioned during interviews or chats with HR, was to review Principal-and-above roles after the candidate accepted terms without telling the candidate or recruiters this was happening, and intentionally delay sending the "formal" contract out so the candidate would believe they had no rights. They decided they no longer had a business need for the role, apparently. You would think a Principal Firmware Engineer already signed up could just have been transferred to any of the 100s of other products, given the implied experience and autonomy from the title, but apparently not. This is a company with about 1600 employees and high technical overlap between products, that was doing well during the pandemic.)
I've read a bit more on this and learned some places (i.e. Google) also insert "Senior Staff" between "Staff" and "Principal", "Distinguished" after "Principal", then "Fellow", "Senior Fellow".
I was officially titled a "Senior Fellow" back in 1998 for my second full time job - I don't think it had the same caché back then :-)