But trying to keep a larger share means you lose out on all of it, as you will get outbid.
If they can save other companies 1.2 million as well (which they will because few companies are so unique) then their price will naturally creep up towards that value as others will be willing to pay more because it's a good deal.
Google receives a huge number of applications from unqualified people. They take the very few that are Google-level.
If you're too good, they will feel threatened and not hire you. It's all politics inside these megacorps. Engineers will never allow real talent to be hired on the same team as them... Why would they compromise their position within the company hierarchy?
They'd rather only hire moderate talent; just smart enough to get the job done but dumb enough to not get in the way of your next promotion.
If the engineer earns you or saves you 1.2 million why on earth wouldn't you be prepared to pay 600k for them?