I think you already read the reason/s but in a monopoly capitalist society, but companies can just smother smaller ones by leveraging their exploited workforce (more output for less cost), out-competing, buying up all competition, much better economies of scale, and access to capital and market forces.

Just take an example of a small business owner who sells sporting goods (I use this example because I love Freak and Geeks lol). How can you possibly compete with Walmart when Walmart has bigger and better inventory, cheaper prices, more locations, basically no competitors, better advertising, etc? Sure lots of people value 'small businesses' from a moral/ethical point of view, but enough for this company to grow and grow and grow and compete with friggin Walmart? That just doesn't happen often.

Now, something like REI, which is a coop, does compete with Walmart in a very niche market. REI has a strong brand and loyal customer base, allowing it to compete effectively in the outdoor and sporting goods sector. However, its focus is more on quality and specialized products rather than mass-market items. Do you think Walmart couldn't just destroy REI if it felt like it was being threatened and it wasn't one of the largest mcap companies on the planet?