I personally see different kinds of ideas:
* The "trivial" ones where there is no risk (we know it's possible) and it "just takes development time". Say I own sport infrastructures and I want to build a system (website, app, ...) where customers can pay, register to some classes, book some infrastructure, etc. In that case, it just takes a lot of development time, and therefore it's expensive. I won't be able to afford that "just for fun".
* The "innovative" ones where there is a risk: I think it would be useful, but it doesn't seem to exist, and I don't know if it is reasonably feasible or not. In that case, outsourcing would mean paying someone to do that research and possibly failing. Many times it will fail (because it's not possible, which explains why nobody does it). If I fail myself, I have learned a lot in the process. If I pay someone to fail, I have just lost my money.
Now it's probably possible to pay someone to develop a product, try to sell this product and then re-invest in the product. But at this point it's not a side project, it's entrepreneurship :-).
To me the whole idea of a side project is that it is something I would like to do, myself. If I was paid to work on it full time, it would become my job and it would stop being a side project. And then other side projects would pop up!
