Because sevens damn fine
It's cheap, swift, and can pinch some powerful spells from your opponents
In other words, your Gruul commander might support some micro-strategy, but aggro beatdown is almost always the macro-archetype
Just to note, there were a ton of bounce spells in Invasion Block, and there are still more to come on this list
At its best, game design inspires worldbuilding, and worldbuilding inspires game design, in turn, in a virtuous circle

7 Island 7 Mountain 8 Forest Aberrant Abundance Acolyte Hybrid Aetherize Arcane Signet Ash Barrens Biophagus Bred for the Hunt Broodlord Cave of Temptation Cinder Glade Clamavus Command Tower Cultivate Deathleaper, Terror Weapon Death's Presence Evolving Wilds Exocrine Exotic Orchard Explore Farseek Frontier Bivouac Game Trail Genestealer Locus Genestealer Patriarch Ghyrson Starn, Kelermorph Hardened Scales Harrow Haruspex Herald's Horn Hull Breach Icon of Ancestry Inspiring Call Magus Lucea Kane Malanthrope New Horizons Opal Palace Overgrowth Path of Ancestry Purestrain Genestealer Rampant Growth Ravener Rugged Highlands Sol Ring Sporocyst Starstorm Temple of Abandon Temple of Epiphany Temple of Mystery Termagant Swarm Terramorphic Expanse The First Tyrannic War The Red Terror Thornwood Falls Trygon Prime Tyranid Harridan Tyranid Invasion Tyranid Prime Unclaimed Territory Winged Hive Tyrant NEXT: Magic The Gathering: Everything You Need To Know About The Commander Format