And the panels synchronize to the grid's frequency; a generator doesn't have any logic to synchronize to the grid's frequency.

Skip to about 10 minutes into this video: https://practical.engineering/blog/2022/12/5/what-is-a-black...

Furthermore, the danger of backfeeding is if you do it in a power outage and you start to power your neighbors' houses. It'll overload your generator and create a hazardous situation for anyone working on the wires. (This is why you're supposed to install either a generator switch or interlock. I have one that only allows me to energize when my main breaker is turned off.)