Software always has much more extreme economies of scale than any physical commodity, and often has network effects on top of that. So it seems like a bad comparison. Do you happen to have any statistics comparing levels of market concentration across different kinds of physical goods?

Sure, it would be equally difficult to start a new company making non-niche[1] t-shirts or toilet paper or plastic cups or light bulbs today.

1 - "non-niche" a/k/a the undifferentiated "standard-quality" product in the middle of the niche. This is not about someone inventing a longer-lasting light bulb or stain-resistant t-shirt. The best analogue to farmers selling commodity soybeans is something closer to plastic food wrap or aluminum foil. Not a lot of small operators playing in or entering those spaces either.