Because historically food is scarce or difficult to obtain, in general organisms develop mechanisms to make good use of it: when excess food ("energy") is available, it is stored rather than wasted.
This is also true in particular for mammals, and for humans. It's quite obvious that humans are very effective at storing excess energy.
It is said that sumo fighters maintain their body mass (muscle + lots of fat) by eating rice (i.e. carbohydrates) and protein.
Lipogenesis (fat generation from carbohydrates) takes place mostly in the liver. "Excess acetyl CoA generated from excess glucose or carbohydrate ingestion can be used for fatty acid synthesis or lipogenesis."
https://courses.lumenlearning.com/suny-ap2/chapter/lipid-met...
The conclusion being: "Humans are great at lipogenesis. That's how we store excess energy."