Because it's generally not the case for early stage employees. The first couple hires, especially on engineering, definitely need to wear a variety of hats. In my first startup engineering gig, I did everything from front-end design and development to backend API work to helping people tech support their Macs and upgrading hardware in team computers, talking with potential investors, etc. Without a doubt, wearing multiple hats is the norm when joining a startup that's pre-seed up to Series A or so.
I'm working at my third startup now (horrible management and lack of understanding of work-related injuries caused me to leave my first two gigs)
I joined each of these startups in their infancy, and at that stage, even if wearing multiple hats doesn't get you hired, it quickly fast-tracks you to a position where you can have more impact in the decision-making process.
These startups were not just in tech either, so I think this is just generally true of startup culture. One was agricultural, one was food service / retail and only my current one actually has me coding, but I still found myself going all over the place with client/customer relations, marketing, spreadsheets, tech support / IT, etc.
What I like about taking these kinds of open-ended gigs is that it can really help provide you the experience and comprehensive knowledge that will land you better administrative / project lead roles elsewhere, as well as prepare you for striking out on your own without becoming overwhelmed.
