* 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.
But people working there are often great.
You can't just hire most of them. So first you pet their ego with some cash & startup success narrative and only then hire.