people are constantly job hopping, Google has to always be hiring just to negate attrition.

Second, there isnt a single budget for hiring that interviews everyone and ships them out everywhere. certain groups/products can be growing within google while others are shrinking. should the chrome team not be allowed to grow because google+ was in the trash can?

third, not every employee is exchangeable, you cant just shuffle everyone around the company. You cant just take someone from the google maps team and have them start working on nexus phones.

all of these things are fairly obvious if you thought about it for more than 30 seconds, so its somewhat concerning you find this topic so perplexing.