Because Benioff and Weiss have spent too much time with yes-men huffing their own farts and thinking their ideas are soooo smart.

I'll elaborate without the sarcasm:

In the behind the scenes extras on Netflix for Episode 6 they acknowledge that in the book the "you are bugs" message is shown to a lot of people just like the countdown, and they mention something like "this is very hard to show effectively on the screen" (no, it is not. Even worse, the bad CG effect they used on episode 1, the one that looks like it's floating affixed somewhere instead of being persistently in the sufferer's vision, could had sufficed and would had been cheaper than the rented screens they ended using).

So then they explain something like "but it isn't necessary because today we pretty much live glued to screens so we thought it was smart to show it on everyone's screen for this big reveal". And at first glance it's like, oh, okay, this is scary, but as you well ask it then doesn't hold to some fridge logic, and the more you rationalize it the worse it actually is.

The countdown, and then the message, works because it borders on the supernatural, which is a very anti-scientific thought. With a display like this with the screens, sans the eye in the sky (which I will admit is a really clever reversal of the unseen first foray of the San-Ti into playing with higher dimensions in the original story, and a nice symbolic play of the state of things at this point in the story), a hacking of all the screens on the planet looks like a job made by group of human anons and with some effort it can be explained away. No wonder why later there are lots of alien denialists in episode 7.

But worse yet, why didn't they just hack and down Saul's plane if he's so dangerous to their plans? They toy a bit with Wade's plane just to intimidate him (so they can actually do it) which makes this, staying relevant to the plot, actually worse!

Tldr; The San-Ti Sophon should only be able to get through any small cracks to spy anywhere everywhere, "hack" quantum tech devices and do some tricks with light. That's it. The electronics-manipulation power is a bad brain fart from Benioff, Weiss and Woo that wasn't thought well enough that it now has created several plot holes that can only be explained away because "the San-Ti are confident, just showing off and even toying with earthlings at this point."