Precisely because it isn't.
Everyone has lots of energy early on, they run out of energy late in games = they arrive nice and early and full of ferocious energy at each ruck + they're faster so if you want your 7 to be a link player and run the ball a bit, he's gotta be fast and agile.
That's Hooper.
20/15/10/5 mins to go, when most players have tackled their heart out (Beale aside), run around a lot and are knackered - you're slower to the breakdown and have less strength and energy when you get there. The last thing you want to encounter in those moments is He-Man latched onto the ball like a limpet crab.
That's Pocock.
Imagine 15mins to go, game in the balance, you've ground you way up field for 15 phases and make it to 30m out from the Wallabies tryline, only to have Pocock (freshly off the bench) latch onto the ball and either turn it over and potentially concede a counterattack try, or concede a penalty that is then NOT GIVEN TO FOLEY TO KICK, but given to literally anyone else, and send you 40m back towards your own tryline.
"Ugh..start again!"
Those kind of events or genuinely sooo depressing, and they break the spirit of a team far more than conceding one 10 minutes into the game.