They didn’t realize they were at war until about 3 months ago. That’s why. It takes time.
Yahoo's: had an offer from Microsoft for ~$33B in 2004 or so, declined, and later hired Marissa Mayer. Eventually sold to Verizon for ~$4B.
HP's: hired Carly Fiorina, Mark Hurd, Léo Apotheker, and then Meg Whitman
Modern boards rarely act in any sort of timely manner.
The former Yahoo assets that Verizon didn't purchase -- Yahoo Japan, plus a large stake in Alibaba, and several other investments -- were liquidated for a ~$40B total return to shareholders over the course of several years. So combined that's a 33% increase over Microsoft's offer. Obviously not a great return over like 15 years, but far from "worst board decision of all time" level either.
Microsoft's 2004 board, on the other hand, I have to wonder what they were thinking...
Seems like you could make an LLM generate product ideas and it would be about equally effective.