The biggest reason is, what is the downside? Working with US spy agencies as an american on american soil you don't risk getting thrown into jail for the rest of your life and the biggest downside is getting caught and fired but you are being compensated with money anyways so why would you care. Working with foreign governments is much more risky but I am sure they have tried and do have agents working in large companies such as google.

who says they haven't? maybe not completely (it's way easier for the US, as most of Google resides on US soil), but you can bet they have people in a position to extract at least some data on demand.

this has previously happened: stuxnet (a product of several intelligence agencies) was digitally signed by a large semiconductor company!