From many, many countries and borders, I find that it's often just down to the customs officer.

Sometimes they're incredibly precise, opening to the first spare page and clearly and carefully stamping the page.

Other times they just open, stamp, and hand it over.

I've got visas on both pages, back to back, facing both directions, in multiple passports.

Most likely, some just find it easier, but your passport has 25/48/50/100 pages for a reason - all can, and do get used.

(although some countries require you have x spare pages left, but that's a different story).

Related:

Can I choose in which page of my passport is a visa going to be stamped?

How do I decide whether to be worried about having enough blank pages in my passport?

What can I do to prevent passport stamps being put on blank pages?