The economics are different because the industries are fundamentally different. Software is never "finished" the way a building is finished. More features can always be added to software. If those new features create new product lines and attract new revenue, then the software engineers' salaries are more than paying for themselves.
But, this obviously carries risk, that the new thing you develop won't be worth as much as you spent. Bending Spoons doesn't want risk, hence their decision.
It is absolutely true that software can be finished, it's just that software appears to be dead if it hasn't had any work done on it for years. You don't need to keep adding features and changing the software ad infinitum.
Just like with your building analogy and with other car analogies presented here, software does need some maintanence every now and again to keep it up to date - with security fixes, compiling to a newer platform, integrating fixes from dependencies, etc. And yes while buildings may be finished they stil require regular maintance if they are used.
I'm not really sure this argument makes sense. Plenty of software I've built is finished, it does the thing I need it to do and I haven't touched it in years. Adding features just because is not a useful way to spend anyone's time, doubly so in a business context.
The businesses they acquire are ones whose revenue has not appreciably grown in many years. They are being sold because the prior owner does not believe they can improve the business any more.
Any profit bending spoons earns they can run off and invest in another business if they like. They don't bother investing in the businesses they purchase because they believe, like the previous owner believed, that there is no more juice to squeeze from that particular lemon.
> Software is never "finished"
Software may never be finished (in your opinion) but the budget of any customer is finite. If you keep reinvesting your revenue forever into "engineering" the product there's going to be a time where a competitor comes in with a finished product matching your customers' requirements and snatches him from you by both charging less and making a profit.