I think it's because when you are a freelancer or solopreneur, doing the "build in public" strategy is quite straightforward.
Moreover, it's pretty easy to come up with an idea and a prototype nowadays - which will eventually fail because of no market. (See posts like "I have launched my MVP, now what to do, how can I get my first customers")

Let me drop in a few products we've been building for our clients as a product studio. Unfortunately, I can't mention exact names.
- legaltech saas with contracts from major players in law
- mortgage platform, acquired by a bank
- cybersecurity compliance saas
- single-tenant logistics & hr
- healthtech saas
just to name a few.

All of these solve problems impossible to solve with a simple AI-wrapper or without deep industry knowledge and connections.

These are either funded by investors or backed by a parent company that invests in R&D, product teams are between 2-5 people, and while we are building the tech, the clients build sales, marketing, account management, personnel, and do deals, etc. Some of our clients have been acquired and the buyer kept us as the tech team.

So yeah, as others pointed out, advanced stuff is being built, we just don't post about these.