Well! They are not just buying but improving it as well. Whatsapp and Instagram are the best example.

Yahoo could not keep up the quality of any product that was acquired by them.

How is WhatsApp “improved” by reducing privacy and adding ads?

End to end encryption was added after the aquisition.

Encryption was already in the works via a collaboration with Open Whisper Systems, the makers of Signal (see signal.org).

AFAIK end to end encryption was actually part of the acquisition deal.

And now they regret it ;-)

While true, I am not really believing Facebook had anything to do directly with that happening. It seems like the entire industry was moving that direction and WhatsApp would've always had to implement it.