It's probably because any good conversion kit would be highly specific to that car model, it'd still require a ton of work to cram in the batteries and do the entire conversion work and still cost a lot. What's the benefit over a fully electric car at that point?

I think there's a pretty good explanation why a lot of electric car conversions are hobby projects, where you don't pay for labour converting the car and the car is generally interesting for the person in order for them to eat the costs.

Because most mechanics aren’t also EEs? I would sooner expect a third party parts manufacturer to start developing them than a mechanic.

Mechanics are naturally curious, they figure stuff out through experimentation. One of the characters in the Electric Koolaid Acid Test, was Hassler. He kept the magic bus going. Or the NZ guy who set a land speed record on a homebuilt motorcycle. Or just about every NASCAR team in the ‘60’s. Motoring is full of characters like these, not formally schooled, but schooled in practice. You don’t need an EE, ME or any schooling at all, you need a willingness to bust knuckles, take things apart put them back again, take them apart, modify them and put them again over and over.

My father was a mechanic and a very good one. I spent plenty of time in the shop growing up. I saw him and his mechanic friends come up with some really ingenious ideas for how to solve problems. I am sure it is what inspired me to get my degree in computer engineering. What I never saw them do was create their own conversion kits and parts. Sure, there are few "mechanics"[0] out there who make stuff like that, but most don't.

[0] These people tend to have a mechanical design/engineering background to begin with.

You're mainly talking about the "old school" mechanics, not the more prevalent ones today who are mostly in it for the money and barely know anything beyond what their certification courses taught them.

This seems like a broad generalization.

It's true, good mechanics are hard to find.

Think Electrician instead of Electrical Engineer.

EEs build the motor and the battery controller. Electrician level skills can arrange the parts within their specifications.

You'd be surprised -- modern cars have a lot of electronics and wiring involved, especially when you start getting into larger trucks and semis.