Having broken a couple of kingpins at high speed on roller skates, it's definitely no fun. Unfortunately, the kingpins are under a lot of stress and they're threaded. Invariably, the break occurs on the threads.

Titanium kingpins are available for skates and skateboards, and are much lighter. I don't know if they help with breakage.

It’s less dangerous on skateboards because they break when landing flat from drops, but it still sucks.

No, steel is harder. Titanium is lighter and very strong for its weight and also very heat resistant, but not as hard as steel

Does it actually break or just come loose? Can loctite help?

When a kingpin breaks it's almost always complete mechanical failure, e.g. it snaps in half one way or another. In addition, it's usually not possible to foresee it happening by visual inspection, the wear and tear is internal.

I think the problem is using better quality materials doesn't really show up or become apparent unless you are already breaking kingpins all the time. It would take a significant marketing budget to convince people to spend 50% more on your trucks for small gains in material strength/quality, and that marketing will quickly wipe out the advantage of potentially higher sales. Will people see it as worth it when the majority of people aren't breaking them often enough in the first place to be concerned with it?

I've been street skating for about 15 years and I've had maybe 2 kingpins break total