1. because there are many small firms producing relatively small amounts. Industries with high fixed costs would be particularly unsuitable to perfect competition.
2. Undifferentiated products is boring choice to consumers such as clothing and cars.
3. Lack of supernormal profit may make investment in R&D unlikely this would be important in an industry such as pharmaceuticals which require significant investment
4. With perfect knowledge there is no incentive to develop new technology because it would be shared with other companied