What is the main reason though? The BigCo being too risk averse? Or employees having too much of a safety net? Or BigCo processes weighing the team down? A combination of these?
Big companies are risk adverse. At the same time, the motivation for the people is missing, in a startup you have the chance to become a billionaire or at least a multi-millionaire, in a big company you can get them a billion and be the employee of the month with a $100 gift card. I saw it myself some years ago when I got my company a ~ 5 million/year saving project all by myself with a one-time cost of ~ 20k, I got a $250 reward on top of the regular salary. That was a lesson to not care too much. 2 years ago I got them over half a million per year in a 3 days of work mini-project, I got nothing, not even a formal thank you note. That project was not regular work, it was none of my business and I came with the idea and the execution alone, so you cannot say I was paid for such work. Translate this into a risk adverse big company play-pretending to do startup stuff.
I don't know if there is a "main reason" for all instances. For me, on this kind of project at one of the Biggest Co's, it seemed a bit like the safety net point you make and it creeps into everything. There didn't seem to be much pressure because the project was never going to be pulled and had infinite money. This seemed to manifest in "good people" (my opinion) working at about 40% capacity and new hires having a background in companies more like BigCo than InternalStartup.
Edit: To clarify my last point, my assumption being that the point of InternalStartup was to allow deviating almost entirely from BigCo's approach to everything and that those people from OtherBigCo would be less likely to have that mindset.
If you work for someone with $80bn of shareholder capital at risk you can take 1/10000th of the risk at work than if you have $8M of shareholder capital at risk.
That depends on what the risk is. If the risk is to the credibility or business model or relationships of the $80bn core business, then sure, the big company absolutely must be more conservative. On the other hand, if only a tiny fraction of that $80bn is needed to underwrite losses from your skunkworks' niche product taking too long to launch, lack of rapid uptake isn't an existential threat (even if the project gets canned, most of the team will still have jobs and they might even keep the profitable part of the project alive) and resources can be shared with other areas of the business, there are other risks that are easier to take.