I’d guess static electricity and heat. Also, we’re always facing it. It’s probably a lot of microbes from our faces, both just lifting off your face and attaching to the screen, but probably more often sneezing, coughing, etc.
Since you’re facing the TV, the TV gets hit. All the microbes stick to the front of the screen, and the heat of the screen helps them grow and multiply exponentially.
Just a guess tho’.
I have a yogurt maker (somewhere – I used to make it a lot). The PERFECT temperature for bacterial growth is something like 80-105F – something like that. The heat of the TV is probably around there.
Since dust attaches to the TV, that’s plenty of places for microbes to hide, too. Little places to attach themselves to while they grow.
It’s like a giant, warm petri dish.
In fact, I ruined ancient tube amp I had (I used to be an audiophile and got a hold of an ancient quadraphonic tube amp) by yogurt. I’d make the yogurt in mason jars on top of it.
One day, one of the jars was too full of milk and spewed over the top into the tubes and everything. Oops.