For the exact same reason that most software companies don't have a Computer Science Ph.D. on staff. Many companies don't need a high-caliber research department.
People who learn and understand how to apply ideas and theories from behavioral economics in biz and econ courses are well-represented in sales and marketing roles. Just like people who learn and understand how to apply ideas and theories from Computer Science in CS courses are well-represented in engineering roles.
> why none of the behaviourial Economists have ever tried starting up?
I'm sure many have lucrative consulting portfolios, but generally speaking: because they already have a pretty cushy and very well-paying job doing what they love.
I've wanted to practice behavior economics meets technological innovation for a long time now - but you're right, there doesn't seem to be a demand for this.
You mean outside the entire advertising industry? Now you might argue that advertising companies are more focused on selling ads to clients than optimizing the ad experience for victims, but it's an entire sector of the economy focused on exploiting the poor decision-making abilities of humans.
Of course, as long as we have had enough understanding that we can affect others, there has been a demand for exploitation through bias.
I think the hope here is that we would create positions that allow BE to benefit others AND businesses by helping people make better decisions. It sounds a bit utopian, but it's certainly not impossible that everyone could benefit from people making decisions that are beneficial to their well-being.