Why not just invest additional resources into more effectively applying current existing control strategies to eradicate these diseases?

Although conventional insecticide-based tools have been a mainstay in fighting insect disease vectors, they have limitations. Insecticide-based tools required continuing reapplication that can be costly to maintain and insecticide resistance is an ongoing problem. Insecticide-based tools historically have been less effective against some mosquito vectors, such as those of arboviral diseases, due to the difficulty of reaching their breeding sites. WHO has taken the position that new tools are urgently needed for vector-borne diseases.

For more information:

https://www.who.int/news-room/fact-sheets/detail/malaria
https://www.who.int/teams/global-malaria-programme/reports/world-malaria-report-2021
https://www.who.int/publications/i/item/9789240015791
https://www.who.int/news-room/fact-sheets/detail/dengue-and-severe-dengue
https://www.who.int/news/item/14-10-2020-who-takes-a-position-on-genetically-modified-mosquitoes