When and why did military spoils change?

I was just reading that the Roman army was actually quite a lucrative career for soldiers, often earning high annual stipends in addition to shares from selling loot, in addition to vast discharge bonuses of dinarii, goods and fertile land. I would assume other ancient civilizations provided similar compensation schemes to their armies in order to generate a large enough volunteer base for campaigns. And thus campaigns became wealth-seeking endeavors, to some extent.

Well, today, the average US soldier earns between $20-50k, below the national average income. And even the most decorated officers rarely earn more than $130k, which is less than the average US professional (MD, JD, PhD, etc.). Why have the pay scales of war leveled so dramatically?

Clearly wars are money-losing endeavors now, and maybe the risk of death in combat has fallen too, but can someone paint a picture of the journey to our current military compensation?