The economics of side projects really underscores the hypocrisy of the corporate ruling class.
First, what is the first thing that a person learns about being wealthy? Diversify. That's one of the most basic principles of financial common sense. Yet it's extremely frowned upon for working people to do the same with their time. If your primary asset is money, you're allowed to diversify. If your primary asset is time and you try to diversify and get caught having a side project, you're "not a team player".
For most corporate jobs, the work itself could be accomplished in 2 to 3 hours per day. People simply don't have enough productive time to fill a corporate workday, a schedule that is based more on availability (often, availability for low-margin uses like worthless meetings) than productivity.
The corporate workday is an international plane ride (8 to 12 hours, mostly sequestered in one location) five times a week. Is this about productivity? No. It's about required loyalty. Even though people only have 3-4 hours of productive time per day, the purpose of the midday plane ride is to dominate so much of someone's time as to maximize the probability of capturing those hours. It's to make it so that the person's side efforts are unlikely to be able to compete, so that diversification isn't (for most people) an option.
The reason people specialize is that there are increasing returns to specialization, as Adam Smith pointed out many years ago.
Therefore, if you specialize, you'll be able to trade your time for more money/bread/whatever. If you want, you can do a little bit of programming and a little bit of ditch digging, and a little baking, and be terrible at all of them, and get paid consequently.
It's not some plot of the "corporate ruling class", whatever that is. Ordinary people diversify their investments as well, mostly via things like index funds, which can be had at very cheap prices.
Different concerns. Specialization is about accelerating (probably exponential) returns to skill growth.
I've had several experiences investing huge amounts of time and emotional energy into companies and projecots that didn't deserve it. Total loss. Skill growth and career advancement I can get behind. Unrequited loyalty means you piss away the only resource that actually matters: time.
Computing is highly specialized, but what we need to tackle that is a real profession. Professionals have ethical obligations that supersede managerial authority, and are both allowed and expected to devote half their time or more into career growth and continued learning, rather than dropping 40-50 hours on short-term, managed work. Are software engineers really professionals? I'd argue that, across the industry, the answer is a resounding "no". But it would be a better world if we were.
Managers don't like rich sub ordinates. There is innate understanding that money must always be paid in accordance with the position with the hierarchy. You are not going to be very rich, diversifying your tiny little investments. But you can be a lot rich if you spend you time well.
Now that's a problem for most managers. Side projects give your ordinary every guy a chance to do something big both in terms of growth and success, in a way the manager can never control.
Its little to do with rationality, or any other economic factor. It has a lot to do with jealousy.