[dspillett]

> why companies as big as AMD do not invest and build what they need in house?

Often it is an expertise thing, especially when buying smaller companies. See https://en.wikipedia.org/wiki/Acqui-hiring

With larger purchases like this one that can still be part of the equation, though there is also the matter of lead times needed to bring a significant team and related infrastructure needed for the project(s) online and up to speed.

Also if a company is seen as ripe for buying, it can sometimes be done in part to stop a competitor getting a chance at the above advantages.

I suspect a mix of all three is at play here.