Lannisters proved that money can't buy good soldiers....
Despite medieval mercenary armies fighting for gold being the best there was in our universe.
They have had several years to rebuild their forces and their men throw down their swords all at the same time. After Dany apparently incinerated 20,000 men from the Golden Company in one blast after destroying all 1,000 ships in the Iron Fleet with like 3. There seemed to be zero resistance to Dany's exhausted forces.
The Lannister Army itself numbered 60-70,000 in the first two seasons, thousands of whom were from Jaime's army that had crushed the Riverlands army were captured in Robb's campaign and would have been released after the Red Wedding. Twyin's main Lannister host was never defeated and fully intact. Where the f-ck are they supposed to be?