I think it's to minimize the damage from the thiefs that hack user's inventaries to stole their items and then sell them.
But, why is a simple email confirmation enough for trades, but not for selling items? Just because they can get money from it?
Pretty sure you dont need to put your mobile number on your settings.
You just need the mobile app to confirm the transactions - at least from what I went through.
Here's my purchases for Steam Summer Sale 2016.......not as much as the previous 3 years but hey, its whatever.
-Saints Row Gat Out Of Hell
-Hyperdimension Neptunia Re;Birth 3 V Generation (and its DLC as well)
-Assassin's Creed IV Black Flag Gold Edition
Total cost: $40