If I hired a carpenter to build a bookshelf for me, I'd be concerned if they started cutting wood before providing an estimate on how much it would cost, and how long it would take to build. I'd be really impressed if, based on my needs, they told me to go to IKEA instead.

I get what you're saying and agree with the sentiment. But as someone who often builds both with wood and software there is one very big difference. Coding doesn't use materials, only time. So quick prototyping costs the same as writing a detailed estimate or writing some other document.

This is something I've come to appreciate more as I've matured as a dev. We spent 20 hours in the last 2 weeks designing out and talking about the hard parts of our next gen infra and delivery process. At the same time, I still believe spikes have value for finding out where the hard parts might be