Answer:
When you let people in the market to freely compete with one another without any restriction, eventually there will be a sole winner who stand in the top of the competition.
When this happen , that sole winner will gradually accumulate wealth and the production process that it structured will become more efficient. This made that winner able to produce the product with really low price and expanded the operation using the gained wealth.
This will make it really hard for newcomers to compete with the existing winner. Eventually, the winner will takes it all and monopoly or will naturally formed (or at least that winner will own majority of the market)
Perfect competition is an effort that people consciously do in order to prevent monopoly. They strictly regulated the competition to ensure that there no difference in price and types of products.
To put it simply, if we try to let free market take it course by itself, monopoly tend to unavoidable. If we try to restrict it without freedom to compete, we technically form a perfect competition.
This is why most markets in the real world fall somewhere between perfect competition and monopoly
