It depends on what they are acquiring.

* People - buying a ready-built high-performing team is cheaper than trying to assemble one from scratch.

* Patents and IP - if it is the idea which is valuable, you don't need the product.

* Competition - the start-up is (or could be) an existential threat. Buying is a cheap and legal way to shut them down.

* Customers - getting the start-up's customers on to your own platform. No need for the old one.

* Ego - buying a start-up is a big deal and helps people feel good. And maybe get a promotion. There's no rationale for buying other than short-term internal metrics. So the product gets terminated quickly.