Since they are a different sort of company, with another legal attack surface.
Going after GitHub means that GitHub has to shut down and the opponent sticks with their legal costs.
When attacking Microsoft they have enough cash to pay you out (in case they lose or they accept a payment to make you shut up) and the case might disrupt Microsoft enough to give you an advantage in a different field.
Not saying that Google would sue Microsoft. Would be very bad for their image. But a risk evaluation is different.
Going after GitHub means that GitHub has to shut down and the opponent sticks with their legal costs.
When attacking Microsoft they have enough cash to pay you out (in case they lose or they accept a payment to make you shut up) and the case might disrupt Microsoft enough to give you an advantage in a different field.
Not saying that Google would sue Microsoft. Would be very bad for their image. But a risk evaluation is different.