Because if they didn’t, as you train troops, your capacity to generate gold decreases.
Yeah that’s kind of a balance I think. Where you need to figure out if the extra troops is worth the less gold. It also means that teaming up can help in multiplayer where you can get someone to supply everyone with gold by claiming locations far from the frontline and never training on them. You could supply them with some Valour and they can supply Gold.
That being said, someday down the track there might be a hero card released that interacts with population later. Right now the only thing you can do with population is use certain cards abilities to return the troops and heros you have back into your hand.