Why don't expert automobile mechanics design engines or are mechanical engineers that design engines by nature expert engine mechanics?

If you are smart enough to dismantle and put back together an entire engine, I think there’s a good chance that you’re smart enough to design one.

Of course, everyone’s life experience is different and some people don’t want to crunch numbers and like working hands on. Then you could also say that being able to put things together well does not translate to the ability to design things at all.

But surely, a person who designs an engine is qualified to repair one. Right?