Why so many suppliers of inexpensive ordinary products
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.