So, your spec house is the yet-another-crud-webapp, where the spec is clear and the tech mature... Engineers do those too, but they aren't so much entrepreneurs as jobbing contractors or agencies
It's fine to move the analogy, but the point remains the same: The more confident you are in your own capacities, the easier it is to bid on a project. And to be confident in your capacities without it being simple hubris, you need to do difficult things that demonstrate to yourself what you are capable of accomplishing.
It's the distinction between building something you've built before and architecting a new bridge or building. The latter is a creative effort with lots of variables that are extraordinarily hard to pin down into estimates that would be acceptable in spec house construction.