Some already are (eg. neem oil and pyrethrins). Many commercial pesticides are also analogs of natural products, many of which were first extracted from plants or fungi.
Re.fungi - largely because they are not very effective. Whereas a few synthetics may have some functionality or structure relevant to a "natural" molecule, it's not commonly the factor in development.
Plant extracts are actually used to combat fungi, as many plants contain natural compounds that have antifungal properties. For example, essential oils derived from plants like thyme, oregano, and tea tree are commonly used as natural antifungal agents. However, the effectiveness of plant extracts in combating fungi can vary depending on a number of factors, such as the specific plant species, the concentration and composition of the extract, and the type of fungus being targeted. In some cases, synthetic fungicides may be preferred over plant extracts due to their consistent potency and ability to target specific fungi.