Why doesn't the crew automatically switch the main view screen to interesting objects?

This happens throughout all the series:

WORF: Captain... Klingon attack cruiser decloaking, bearing zero-one-zero mark three-two-seven.

PICARD: On screen.

.

PICARD: Report.

RIKER: There's an object of some kind closing in on our position.

PICARD: On screen.

.

WORF: We are being probed.

RIKER: What's the source of the probe?

WORF: A ship. It is on an intercept course.

PICARD: On screen.

.

KIRA: We have a Cardassian warship approaching. Fast.

SISKO: On screen.

.

TUVOK: I'm picking up a vessel.

JANEWAY: On screen.

---

What is on the view screen prior to the order to show the object of interest on the screen?

Why does no one in Star Fleet just instantly switch the screen, without being asked?

I realize a likely answer is for dramatic effect, but as I just recently re-watched TNG this bugged me a bit and I'm wondering if there is actually an in-canon explanation.