Some problems with that:

Why should I personally pay for the tools I use at work to make my employer money? Should I also buy my own work laptop? My own software licenses? It doesn't make sense.

Then there's also the tax issue, as others have pointed out. I'd be giving post-tax money in a lot of instances, whereas for Google it would have much more favorable tax treatment.

But the biggest problem is that me personally making $1000/year of donations is pissing in the ocean. It accomplishes nothing. A program where every engineer is doing so accomplishes a lot more.