There's a big difference between a "prototype" (or a POC, or a spike, or whatever your company calls it), and an "MVP" (minimum viable product). An insecure product is not viable. A product which cannot be extended or maintained without being almost competitively rewritten is not viable.
MVP means just enough engineered code to solve a problem, rough around the edges and lacking features sure, but not built by someone who has literally no idea what they were doing.
Prototypes of physical products are never put into production and sold to consumers. Unfortunately software prototypes "run", and are sold at that point. Then they begin to scale, and the inherent flaws in their design are amplified. The same thing used to happen with MS Access apps; the same thing still happens with "low code" solutions.
The engineers cost just as much after the prototype phase, but if you don't hire them to build your MVP then you never have one.