Why so many suppliers of inexpensive ordinary products There are a very large number of suppliers of common, inexpensive products, and I am wondering why a few large companies don't simply provide all the supply? I am not thinking of food or clothing or something where people could meaningfully differ on preferences, I mean things we don't see or can't distinguish anyway, things that I call 'commodities'. The best example I have today is cabin air filters for cars.
I went to buy one at Walmart today, and there were several brands and about 50 different sizes, and all the brands had a different numbering scheme. This was vexing, and I bought one only to discover that it didn't fit. I looked at Amazon.com and found an absolutely staggering number of brand names. Is there some point to this? Are cabin air filters so exciting that everyone needs to get in on the action? Why don't a few companies make them all? Is there a reason that the 'marketplace' can support a huge number of producers for something no one could tell apart or really care much about?
Perhaps this relates to the concept of Indifference in economics. I have seen this dynamic in other products, but this is the best example I know of. Try looking on Amazon and see for yourself.
There can be several reasons or combination of reasons that could be responsible. This is too broad topic to go into every minutiae detail but main well known reasons from industrial organization literature would be (see Peitz and Belleflamme: Industrial Organization: Markets and Strategies):

Small businesses might have cost advantage over large businesses. Not every industry has production function with increasing returns to scale. If returns to scale are decreasing there might be an optimal sweet spot size for a firm beyond which it does not pay to grow output more.
If there are no significant barriers to entry, markets with homogenous products will typically attract a lot of competition.
Government policy might be responsible for that as well. Many countries support small and medium size businesses with various policies. If government positively discriminates towards such businesses with lower corporate taxes, subsidies etc, then this might discourage expansion of these businesses since loosing subsidy is equivalent of facing tax on a margin and thus it would discourage businesses from expanding unless there would be sufficiently large economies of scale that would outweigh this.

Furthermore, note you are likely getting causality in a reverse. It is well established that competition among many suppliers of similar/same product leads to lower prices in most models (see overview of industrial organization literature in the above mentioned Peitz and Belleflamme). Consequently, the reason why common items are cheap is that there are many suppliers in the first place.