The people at the "tech giants" who would be involved in such a buying decision are by and large located in Silicon Valley, and by and by likely to hear about things happening in Silicon Valley.
Furthermore if a tech giant buys you, they are hoping that you will grow. This requires making hires that are easier if you are in Silicon Valley. Questions about your prospects for making those hires somewhere else limit the top-line growth that they can hope for.
Moreover if they wind up buying something away from Silicon Valley, they know that there is likely to be less competition for the company, so they can probably get a better price for the same prospects.
In addition, talent acquisitions rely on the high cost of getting engineers. That means there has to be high demand relative to supply. In SV there is high demand right now even with a large supply, allowing talent acquisitions of $1.5m per dev.
Furthermore if a tech giant buys you, they are hoping that you will grow. This requires making hires that are easier if you are in Silicon Valley. Questions about your prospects for making those hires somewhere else limit the top-line growth that they can hope for.
Moreover if they wind up buying something away from Silicon Valley, they know that there is likely to be less competition for the company, so they can probably get a better price for the same prospects.